Bangkok is a must.
Also Chiang Mai is awesome (in the north) for night markets,scenary and a cooler atmosphere. Pattaya is great even though a bit cheesy and sleazy but still cool and good buzzing night lige. Also try krabi for peace and tranquility and awesome beaches.
You will need at least 4 days in Bangkok, 3 days in Chiang Mai + 5 days in Pattaya/beech
I went there for my honeymoon about 4 years ago and it was truly magical. Still think about it now, since then we have been all over the world but Thailand is still my favourite for its balance of shopping,adventure,relaxation,food and people.
We did it first class but you can also do it on a shoe string.
I would probably suggest booking a package as they usually include all the excursions you would ever need and also 'own time'.
We always use Kuoni when doing long haul because of the quality of both service and well planned trips. How much are you looking to spend?
checkout http://www.kuoni.co.uk/holiday/Far_East/Thailand/index.html
and download the 'world wide 2007' brochure (top left) even if you dont use them it gives you loadsa insight into thailand http://www.kuoni.co.uk/brochures/index.html