Here's the tough part. There are so many things to see and do a country like this that I'll just list some that I've done. It is well beyond the scope of a web site to describe each, but there are dozens of books devoted to "What to do in Thailand." Keep in mind that English is spoken in most areas throughout the county, and signs are written in both Thai and English.
(The best place to start)
Visit the Grand Palace.
Shop for world renowned bargains, including having all your clothes custom made for you from cotton or silk material you pick out.
Go to the Floating Market (shop and eat - all by boat).
See the national sport, Thai kick boxing, live at ringside.
Visit Kanchanaburi to see the famous Bridge on the River Kwai.
Snake farm / Crocodile
Rubber canoes trips take
you through caves leading to fabulous lagoons in the Andaman Sea.
Elephant trekking through jungles hiding huge waterfalls.
Remote trekking for the hiking die-hards of the world.
If clubs are what you're into, Thailand has the wildest clubs you've ever seen. Taxis are so cheap, getting anywhere you want to go is never a problem.