Bangkok is at the start of many visitors' itineraries, and while a modern city, it has a rich cultural heritage. Most visitors at least take in the Grand Palace, a collection of highly decorated buildings and monuments. It is home to Wat Phra Kaew, the most sacred Buddhist temple in Thailand that houses the Emerald Buddha. Other cultural attractions include Wat Pho, Wat Arun and Jim Thompson's House, but these are just a fraction of possible sights you could visit.
The former capitals of Siam, Ayutthaya and Sukhothai, make excellent stops for those interested in Thai history. The latter could be combined with a visit to Si Satchanalai and Kamphaeng Phet, all of which are UNESCO World Heritage Sites. Khmer architecture is mostly found in Isaan, with the historical remains of Phimai and Phanom Rung being the most significant.
In the northern provinces live unique hill-tribe peoples, often visited as part of a trekking. The six major hill tribes in Thailand are the Akha, Lahu, Karen, Hmong, Mien and Lisu, each with a distinct language and culture. Chiang Mai makes a good base for arranging these treks, and has some cultural sights of its own, such as Wat Doi Suthep.
Kanchanaburi has a lot of sights related to World War II. The Bridge over the River Kwai, popularised by the film of the same name, is the most famous one, but the museums in its vicinity are a lot more moving. "The Dead Railway" (tang rod fai sai morana) is the railway constructed by captive allied soldiers during World War II. This railway has a nice view all along its route.
Thailand's beaches and islands attract millions of visitors each year from all over the globe. Hua Hin is Thailand's oldest beach resort, made famous by King Rama VII in the 1920s as an ideal getaway from Bangkok. Things have considerably changed since then. Pattaya, Phuket, and Ko Samui only came to prominence in the 1970s, and these are now by far the most developed beach resorts.
Krabi Province has some beautiful spots, including Ao Nang, Rai Leh and the long golden beaches of Ko Lanta. Ko Phi Phi, renowned as a true island paradise, has been undergoing massive development since the release of the film The Beach in 2000. Ko Pha Ngan offers the best of both worlds, with both well-developed beaches and empty ones a short ride away. It is also where the infamous "Full Moon Party" takes place.
Ko Chang is a bit like Ko Samui used to be. It has a backpacker vibe, but is fairly laid-back and there is accommodation in all price ranges. If you're looking for unspoiled beaches, Ko Kut is very thinly populated, but also difficult to explore. Ko Samet is the closest island beach to Bangkok, but its northern beaches are quite developed and hotels are pretty much sold out on weekends and public holidays.
While not as beautiful as Malaysia or Indonesia, Thailand does have its fair share of tropical forest. Khao Yai National Park, the first national park of Thailand, is the closest to Bangkok. Wild tigers and elephants are increasingly rare, but you can't miss the macaques, gibbons, deer, and species of birds. The stretch of jungle at Khao Sok National Park is probably even more impressive, and you can spend the night in the middle of the jungle.
Waterfalls can be found all over Thailand. The Heo Suwat Waterfall in Khao Yai National Park and the 7-tiered Erawan Falls in Kanchanaburi are among the most visited, but the Thee Lor Sue Waterfall in Umphang and the 11-tiered Pa La-u Falls in Kaeng Krachan National Park are equally exciting. Finally, the gravity-defying limestone formations of the Phang Nga Bay shouldn't be missed by anyone who stays in the region.