You have lots of choices in Thailand for a vacation and you need to figure out exactly what you plan to do while you are there.
Your first stop will be in the capital, Bangkok and you should plan to stay there for a few days, especially if this is your first visit to Thailand. There are river boat dinners, short day trips to beautiful temples, and longer jaunts to the old capital, Ayudhaya, or visit the site of the Bridge on the River Kwae in Kanchanaburi for some WWII history.
If you like the beach, you have numerous choices. You can go to Pattaya, which is about 1 hour drive from Bangkok, or you can catch a short flight to Phuket or Koh Samui. If you really want to see pristine beaches and emerald water, go to Krabi. You can dive, snorkel, or swim to your heart’s content.
If you prefer cooler weather, head up north to Chiang Mai or Chiang Rai and visit the temples and hill-tribe people. Local handicrafts, the Golden Triangle, and more await you in Northern Thailand.
For a very laid back vacation (my favorite), head to the northeast to an area referred to as Isaan (or Isan, or Esarn). This is a an area that includes Udorn Thani, Korat,...