Bangkok is the usual gateway for most travellers, and you should plan on three or five days in the capital. This should allow sufficient time for seeing the major sights such as the Grand Palace and Temple of the Emerald Buddha, Wat Arun, Wat Pho and Vimanmek Mansion.
Perhaps also a trip along the Chao Phraya River or a shopping jaunt to Chatuchak Weekend Market. You might also take an out-of-town excursion (Ayutthaya or Kanchanaburi are top choices).
If a beach vacation is your priority, Bangkok could be followed by a week at Phuket or Samui Island, which offer the most exotic settings. Should your time be limited, the resorts of Pattaya, Rayong, Cha-am and Hua Hin are closer to the capital.
For a different scene, northern Thailand affords plenty of scope for both cultural sightseeing and refreshing escapes into the countryside. Chiang Mai is the best base for exploring the region, offering its own sights as well as numerous hiking and trekking options, and a stay of three or four days gives a good introduction.
If you have more time, overnight visits to Mae Hong Son or Chiang Rai are well worthwhile.