In reply to Doad13:
I think you've hit the nail on the head already, with Vietnam. Do an open jaws ticket - arrive in Saigon and leave from Hanoi (or vice versa), and 3 weeks would give you just about enough time to go from one to the other without feeling too frazzled. Brilliant food, gorgeous landscapes, close contact with the coast much of the time and do-able independently or as a package.
Just make sure that you're not flying into and out of the same city - that would be a complete arse, as you'd have to do a mammoth amount of backtracking.
I'd have to admit that I've not been there in a good few years, but if you have half as good a time there as we did, you'll have a blast. We had a month, and whilst we would have liked to stay longer, we weren't at all rushed.
Martin