Everyone will tell you 2 weeks is not long enough, but it can be done! Our first trip we travelled on a friday, arrived sat 6 am, 2 nights at the Marriott River Garden (get a river view room), then had a prebooked 6 day tour from premier holidays which was 2 nights Mae Hon Song, 2 nights golden triange, 2 nights chiang mai full board with a guide. this was a great introduction to the country. We then booked 5 nights in Phuket (but samui or krabi would do just as well), then returned to Bangkok for a night and returned home (to heathrow). this worked out very well and you get to see a lot. Over time we have moved on to other things in thailand but this was a wonderful introduction to the country. good luck!

Or, 2 nights bangkok (try the silom serene for much cheaper than marriott!) 3 nights chiang mai, then either phuket or samui, or both, for 7 nights finishing with a night in bangkok before flying home.