I would go to Bangkok and Phuket (or Ko Samui) as my wife and I did during
our one week visit to Thailand last year. Why Phuket? It provides a welcome respite from the hustle and bustle of Bangkok and beautiful beaches.
Whatever you do, please consider using Priceline (and/or Priceline Hong Kong and Singapore) to secure ridiculously inexpensive hotel rooms. My wife and I absolutely loved the Bangkok Marriott Riverside, and even more so at $47 per night.
Bidding For Travel and the
Priceline Asia Discussion Board will give you the data you need to bid intelligently.
If you travel outside Bangkok via air (whether to Chiang Mai, Phuket, or somewhere else), it is definitely worthwhile to purchase a business class ticket on TG. The premium over coach was only about $25 roundtrip when we went. Check out the Thai forum for additional information.
And you will have no problem locating a taxi at all hours of the night. The great thing about Thailand is that everything is dirt cheap (comparatively), including taxis. Enjoy your trip.