Thailand is fantastic - three days in Bangkok is enough though. Do the tourist sites - Royal Palaces, Zoo, Chaopraya River, Temple of the Emerald Bhudda, The night market and even the Red Light area is a tourist attraction in itself. Take a trip up to Konchaniburi to the Bridge on the River Kwai and the beautifully kept Commonwealth War Cemeteries.
Chaing Mai for the scenery and meeting the Karen hill tribes people etc...the people of Thailand are amazing as is the scenery and the wildlife.
Pattaya is single bloke heaven. Bars, Clubs, Girls, Great food, Cheap beer but, like anywhere in the world, take care and you'll be OK. Have a massage or two (without the sex bit included), have a shave/facial/head massage - about £3 the lot!
Don't bother taking a lot of clothes - they are so cheap to buy out there it's less hassle than buying here to take.
A word of warning though. Do NOT fuck with the Thai police. They are serious dudes and the prison system doesn't cope well with foreigners. Drug possession is a BIG no-no as is, and rightly so, the Child sex trade. A Reviewiend of mine works for the Foreign & Commonwealth office and worked out there for four years and all too many tourists try to think they are smarter than the locals but they aren't and fall foul of the law.
Also - don't disrespect the King of Thailand or the Royal family. You can got to jail for that!
Apart Reviewom that enjoy it's a great place with super culture, a chilled out people, great food and great weather.