You have many options for a holiday in Thailand and you need to find out exactly what you plan to do while you are there.
Your initial 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 trips to the ancient capital Ayudhaya, or visit the website of the bridge on the river Kwae in Kanchanaburi for some WwII history.
If you like the beach, there are plenty of options. You can go to Pattaya, which is about 1 hour drive from Bangkok, or you can take a short flight to Phuket or Koh Samui. If you really want to see unspoiled beaches and emerald green water, go to Krabi. You can dive, snorkel or swim to your heart’s content.
If you prefer cooler weather, head north to Chiang Mai or Chiang Rai and visit the temples and hill tribes. Local arts and crafts, the Golden Triangle and much more await you in northern Thailand.
For a very relaxed holiday (my favourite), head north-east to an area known as Isaan (or Isan or Esarn). This is an area that includes Udorn Thani, Korat, Khon Kaen, Surin and several other smaller towns and villages. The pace is slower, the people are friendlier and the cost of living is about 50% lower than in the tourist areas.