Thailand is a bigger country than most people think. Anybody wanting to go there and see all the nooks and crannies in one go will quickly find that you can use up your entire vacation in Southeast Asia exploring just this one country. That is why your time would be better served by prioritizing the more well-known sites. For a first timer, your first stop is Bangkok.
With three days in this sprawling metropolis, you can explore most of the cultural landmarks, such as Wat Phra, Jim Thompson’s House, Wat Arun, Chinatown and the National Museum, but you have plenty of time to spare with loads of amazing shopping centers. For the high-end stuff, there are huge shopping complexes such as the MBK and Emporium, or you can explore the traditional local shops such as the Night Bazaar, the Chatuchuk Weekend Market and the Pratunam Market.
As many Thais speak English, do not feel that you have to limit your trip to just the major urban hubs like Phuket. Grab a bus and head over to the Andamom coast for some terrific island hopping and exotic beach life. Here, you can take on as many water sports as you please while enjoying a relaxed atmosphere.
If you do choose Phuket, down south is the famous Krabi province where you can choose to stay on one of many great islands, from quiet Ko Muk to pumping Ko Lanta, where things are a bit livelier.
Finally, Chang Mai is a first timers must, with a history and cuisine culture second only to Bangkok, and with more than 30 temples, Chang Mai affords plenty of culture.