The attraction of Thailand is being able to take it easy, soak up the Thai way of life and check out the odd temple or interesting sight -— it's definitely not the place for a Euro-style dash from tourist attraction to tourist attraction. So it's probably best not to try to cram too much in.
Thailand in 2-3 days
You could easily spend the full time just in Bangkok, but after a day doing the sights in Bangkok (Wat Pho temple and the reclining Buddha, and the enormous Chatuchak weekend market) we and checking out the nightlife, take a day trip to the less chaotic, and historic and cuturally-rich city of Ayuthaya.
In 7-9 days
Spend a couple of days in Bangkok then head either north or south, by plane, train or bus. If you want to take it nice 'n' easy, head down to one of the islands (Ko Samui, Ko Pha-Ngan or Ko Tao) off the east coast for 3-5 days. If you'd rather get some culture and a bit of adventure, head north to Chiang Mai or Chiang Rai, from where you can organise trips to outlying viilages and towns and jungle treks.
Two weeks +
If you've got enough cash for interrnal flights, in a couple of weeks you could see Bangkok and both northern and southern Thailand. Alternatively, after Bangkok do a round trip to Kanchanaburi, where you can get a taste of village life, elephant treks and the Thai-Burma railway, before heading further north to Sukothai, a base for exploring the World Heritage listed ruins of ancient cities.