Thailand forum

I posted this on the Laos Board, but figured I might get some different perspectives on the Thailand board:

Heading to SE Asia for 3 weeks with my girlfriend and looking to see the best sights. I realize we are "flying" through, but we don't get enough vacation time. She wants Halong Bay, I want Luang Prabang and we both want Bangkok and Angkor.

Is there anything here that stands out good or bad? We want to see breathtaking beauty and cool sites. I am a history buff and she loves nature. She would like to exchange a flight for a cruise somewhere, but I don't see how to possibly do this.

Also, can anyone recommend any good day tours?
Thanks for any feedback.....

Arrive Bangkok
Bangkok
Bangkok (Ayutthaya)
Bangkok (Kanchanaburi)
Bangkok
Leave for Phnom Penh --Thai Air
Phnom Penh
Leave for Siem Reap --(bus) Mekong Express
Siem Reap
Siem Reap
Leave for Luang Prabang --Lao Avaition
Luang Prabang (Elephant Village)
Luang Prabang (Kung Su Falls & Pak Ou Caves)
Leave for Vientiane --Lao Avaition
Vientaine
Leave for Hanoi --Lao Avaition
Hanoi
Hanoi
Hanoi
Leave for Bangkok --Thai Air
Leave for Home