The following is a random collection of impressions based on visits of varying lengths to these places. It's not intended to be a comprehensive listing, and the size of the entry does not necessarily reflect the attractions of the place in question.
These are just a few of the most obvious things to see in and around Bangkok. Travel agents will run excursions covering many of them, and some of these can be good value. But for the Grand Palace, Wat Pho, and Vimanmek Palace, you will spend long enough at each that it's probably better to take a taxi. See the section on transport for the best way to get around.
Your first stop should be the Grand Palace and Temple of the Emerald Buddha, Wat Phra Kaeo. If possible, go on a weekday, as some parts are closed at weekends. But if there's no choice, go anyway. Admission (about 200 baht) includes tickets, valid for a month, to the Coin and Medal Museum (on the same site), and the Vimanmek Teak Palace complex (which will take another half-day at least.) On entering the Palace from the ticket office, on your left is a sort of cloister where there should be signs giving the times of (free) English-language guided tours. It's well worth taking these. The Coin and Medal Museum is a drab name for what is really the Crown Jewels (and it's air-conditioned) so it's doubly worth visiting after you've finished the main palace tour - as you exit from the northern end of the palace complex, keep turning right until you return to the main entrance.
The Emerald Buddha (actually made of jade) is Thailand's holiest object and must be treated with great respect. Kneel (don't forget to point your feet away) in the temple, and remember that photography is absolutely forbidden. Offenders have their film ceremonially shredded. (Note that "professional" photographic equipment is also banned on the Grand Palace site - don't take anything bigger than a 35mm still camera or a "domestic" camcorder.)
Next door to the Grand Palace is Wat Pho - visit it next. This boasts some bizarre architecture and the enormous Reclining Buddha. Along the side of the Buddha are 108 begging-bowls. It is considered to earn merit if you put a 25-satang coin in each one. Also on this site is the leading school of Thai massage. If you're feeling in need of some vigorously relaxing therapy, this is the place to go.
The Vimanmek Palace grounds contain the old teak palace and a number of collections of royal memorabilia, the King's collection of photographs, the gifts given to him by heads of state at his jubilee, and a museum of Thai craftsmanship. This site is some way out of town, and not all taxi drivers know it. It's near the Dusit Zoo - ask for this if the driver is confused.
There are many other temples to visit - it may be worth taking an organised tour of them, to avoid too much negotiation with taxi drivers. Wat Traimit contains a 6-ton solid gold Buddha, and is a good starting point for a walk through Chinatown. Wat Saket is next to the Golden Mount: it's a long walk to the top of this artificial mountain, but the panoramic view of Bangkok is worth the effort.
Wat Arun, the Temple of the Dawn, is on the opposite side of the river from most of the other sites, so it's best to visit this one by boat. Look out for the decoration made from tiny pieces of broken ceramics. Much of this came from everyday crockery donated by its owners in order to make merit.
The National Museum has a comprehensive collections of things Thai, and is well worth a visit.
Jim Thompson's House. This is a traditional-style teak house built for the legendary JT. He was in the CIA and retired to Thailand, where he almost single-handedly revived the Thai silk industry before mysteriously disappearing. The house is open to visitors.
Further afield, most travel agents offer various one-day tours. The agents all cooperate and consolidate, so you may well find yourself travelling with a completely different company. These tours include pick up from your hotel by air-conditioned minibus, which will either take you to your destination or to a central point where you join a larger coach party. Don't lose your vouchers! Typical tours are the following:
Kanchanaburi and the River Kwai, including the war cemetery, the museum, a trip on the railway, lunch at a hotel, and a river trip by longtail boat.
Another popular one-day tour is to the royal summer palace at Bang Pa-In, continuing to the ancient capital Ayutthaya, and returning by riverboat, with a lavish buffet lunch on the boat.
For a one-day do-it-yourself excursion, take the Chao Phraya Express river-bus to its northern destination, Nonthaburi (Thailand's durian capital - note the decorations of the lamp-posts on the promenade) and enjoy lunch at the riverside restaurant.
Another do-it-yourself trip is to the Chatuchak weekend market. This is easily an all-day excursion in its own right. The easiest access is via the Metro or Skytrain.
Within the city walls the main places of interest are Wat Phra Singh, the principal temple, and Wat Chedi Luang, whose enormous chedi was partly destroyed by an earthquake in the 126th century. Outside to the east Wat Bupparam is on the main commercial street leading to the night market. This road extends for 13km through a variety of villages housing workshops and factories for all the local crafts. To the west is Wat Suan Dork, with a collection of chedis containing the ashes of the Chiang Mai royal family. Here you will also find the Hill Tribe Products Foundation, with their craft products on sale.
To the northwest the city is dominated by the mountain of Doi Suthep, topped by Wat Phra That Doi Suthep. This is the North's holiest Buddhist shrine, and not to be missed. Fixed-fare songthaews between the city and Doi Suthep start from just opposite the northern city gate. The fare is 30 baht up, 20 down. From where the songthaew stops, it's 300 steps up to the first terrace. Note the naga (snakes to repel demons) flanking the steps.
In the same direction, set on the side of the hill leading up to Doi Suthep, is Chiang Mai Zoo, which has a large collection of animals, mostly kept in large areas approximating their natural habitat.
Chiang Mai is a major centre for hill-trekking: the travel agents offer anything from day trips to the Elephant Training Centre (includes an elephant ride and bamboo rafting) to serious excursions of up to 15 days.
At the heart of the Golden Triangle, Chiang Rai is another starting point for trekking, or for visits to Burma or Laos: Chiang Khong, the nearest border crossing into Laos, is a few hours away by a leisurely public bus, and from there you can cross the river to Huey Xai in Laos, and take the slow boat down the Mekhong to Luang Phabang.
The town itself has an interesting collection of wats and yet another Emerald Buddha.
A typical Thai provincial town in the northern plains, sandwiched between the railway and the river Nan. A stroll through the riverside night market, and a riverside dinner, is a good way to end the day here. Apart from the usual collection of temples, the biggest attractions are Sergeant-Major Thawee's Folklore Museum and, just along the road, his Buddha foundry.
Being on the railway from Bangkok to the north, Phitsanulok is a good place to break a journey to Sukhothai, continuing by bus after a night's stay.
This is another ancient capital, which has been restored (detractors say over-restored) as a historical park (or theme park?) It covers a large area, and you will need to hire a bicycle or other transport, and spend two or three days looking around the impressive collection of architecture. Various combinations of tickets are available, depending on how long you wish to stay, which areas to visit, and your mode of transport.
Most people choose to stay in New Sukhothai, 15 minutes away by somgthaew, but there is a guest-house within the historical park itself. If you dostay in Old Sukhothai, be prepared to dine early and spend a quiet night. The facilities are mainly aimed at daytime visitors and everything stops at sunset. There is no night market, but some of the food shops do run a take-away service for the locals and will rustle up a dish of the house special even when apparently closed. I enjoyed a particularly memorable dish of fried rice at one of these shops; when the cheerful lady in charge discovered that I preferred it "Thai-style" she prepared a second helping, this time with all the proper spices.
Unless you're here for the water sports or the plentiful carnal attractions, or maybe escaping form Bangkok for a few days, Pattaya is perhaps not the best way to experience Thailand. The town is an overdeveloped holiday resort thronged by obese Europeans; the restaurants are more likely to have menus in German or Russian than Thai; the sea is polluted (though the beach itself is clean.) Nevertheless, it has a wide variety of sporting and shopping venues, a thriving night-life, centred on "Walking Street", an excellent fish restaurant on stilts over the water, and a variety of entertainment, some suitable for a family audience; the transvestite cabarets are particularly noteworthy.
Jomtien, just round the corner, is a better though more expensive place to stay, and still within easy reach of the Pattaya night-life. Regular songthaews ply between the two.
The next best thing to a desert island. The virgin centre of the island is surrounded by beaches, each with a cluster of hotels, bars and restaurants. Most hotels run their own boats from Ban Phe on the mainland, or you can take public transport to Na Dan on the northern tip of the island.
Local boat-owners run excursions to other nearby islands, but (unlike most other places) there is no system of travel agents here to consolidate bookings. This is a drawback for the single traveller, as you need to make your own arrangements to form a party large enough to justify the hire cost.
One of the most beautiful parts of Thailand, and consequently a bit of a backpackers' ghetto. On the Andaman coast of southern Thailand, its spectacular karst scenery, beaches and islands are well worth visiting. There are a number of resorts scattered along this stretch of coast.
Note that a significant proportion of the people here are Muslim, so everything stops for the end of Ramadan (much like Chinese New Year further north.)
I stayed for a few nights at the Krabi Resort, which is not in Krabi town but at Ao Nang. This is expensive but beautiful, with bungalows dotted around a tropical garden. Watching the sun setting through a tropical downpour from my balcony was worth the expense. After that I found cheaper accommodation just off the beach road in Ao Nang.
Local travel agents run a variety of excursions, both snorkelling trips to nearby islands, and inland ones, such as to Wat Than Seua, which is 1272 steps up a mountainside, and a natural sauna fed by hot springs.