The largest city in Thailand is Bangkok, the capital, there is an order among the chaos of the hot dirty streets some high class shopping and a futuristic SkyTrain. Stay in some of the best hotels in the world at affordable rates.
Check our own review of Le Meridien in Patpong
What to do in Thailand: Trekking, Scuba Diving, this is one of the cheapest countries to learn to do your PADI course in Ko Tao. The Beach - love the film ? Of course you did you are never far from a fantastic beach in Thailand nor a bungalow on the beach, Fantastic Temples, Amazing food. Highlights are Scuba Diving in the Andaman Sea, Ko Phi Phi, Ko Lanta, Climbing in Krabi , Railay Beach, James bond Island. Do read the Bangkok Hilton and stay clear of drugs, there is no tolerance of them in Thailand. Visit River Kwai Bridge at Kanchanaburi , the nearby museum jeath museum. Chiang Mai and Chiang Rai offer elephant rides trekking and rafting. Drink snakes blood and dance till dawn at the famous full moon party in hat (haad) Rin.Rin is two beaches separated by a narrow quaint street:The sunrise on the east and sunsets on the west. Sunrise beach is a wide stretch of white sand in a stunning curved bay while sunset beach is not really as nice.

Thailand has a good infrastructure compared to some countries all you to get about in reasonable comfort and never have much more than a 12 hour trip. Use the budget airlines like air asia however and you may never spend more than a half day of travel. See this 2 week Thailand Tour.
Another way to see Thailand Backpacking

Countries and Destinations Near Thailand

Burma

Laos

Cambodia

Malaysia

Indonesia