I went to Chiang Mai , Bangkok and Phuket for my visit there.
Chiang Mai is very beautiful and so tranquil. Took on an Elephant Safari as well bamboo rafting. Also there is a beautiful Orchid Garden which also filled with rare and beautiful butterflies.
There is also a textile museums and large jewelry museum/shop. Also I visited honey farms and leather farm that specializes in sting ray and elephant leather.
There is another museum that documents and display royal garments of the past.
Not to mention one of largest Buddhist temple north of Chiang Mai.
Bangkok floating market and night market are some of the places to check out for bargain goods. Dont forget to visit Pat Pong or 'Ping Pong' ;) for night life in the red light!
Visit the former palace of the emperors of Thailand. HRH main palace is close by but I don't remember if its open to public.
Jim Thompson museum is another place to see. Oh and for shopping.
Gaysron shopping plaza has all the high end goods, the other shopping places to see are all of Paragon shopping centres are the places to shop the good stuff!
Phuket and the islands are great..The Full Moon parties is the place to be.