Thailand's attractions that you cannot possibly see them all on one visit. Planning your trip requires careful consideration, and it is best to tailor an itinerary to specific interests such as the beach, in preference to national parks and the countryside.
Bangkok is the usual gateway for most travelers, and you should plan on three to five days in the capital.This allows sufficient time for seeing the major sights, taking perhaps one out-of-town day excursion (Ayutthaya or Kanchanaburi are top choices), and exploring Bangkok's dinning and shopping possibilities. If a beach vacation is your priority, Bangkok could be followed by a week at Phuket, Krabi or Samui Island, which offer the most exotic settings. Should your time be limited, the resort Pattaya and HuaHin 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, with a stay of three or four days a good introduction. If you have more time, overnight visits to Chiang Rai is well worthwhile.