My sister's travelled extensively in the area, but I've just done Siem Reap, Phnom Penh & southern Vietnam. Halong Bay is more than just a beach, but a whole bay of beaches. Though people go for the bay, and not the beaches. Apparently it's insanely touristy (I think you might be hitting the beginning of the peak season?), but worth it if you can get past that.
Luang Prabang is where people go for tubing. Or went, I should say, because they banned it when people kept dying. People kept dying because everyone there is completely hammered, stoned, or most commonly, both, and mixing that with rapid water isn't a good idea. So perfect if you like the whole backpacker partying thing, but I can't think of anything more tacky, to be honest. I guess it might have some non-tubing things too, especially now.
Siem Reap is a fantastic place, mostly for Angkor Wat. There are other things around, though, like a good landmine museum, and it's also a really nice place to chill out and drink mango smoothies. And beer (the two main brands being Angkor and Anchor, IIRC).
Phnom Penh isn't so fantastic, but is eye-opening and has some nice things. It's quite seedy, though, and is known for British sexpats. I've been to 44 countries, and crossing the road there is probably my most intimidating experience. Ho Chi Minh City on the surface appears more difficult due to wider roads, but is full of two-wheeled vehicles, while in Phnom Penh you're stepping out in front of expensive SUVs (corruption). They have a harder time dodging you, and less incentive.
The bus ride from Siem Reap to Phnom Penh was the most exhilarating experience of my life, BTW, though that certainly isn't what I want out of a simple trip from place A to place B.
Ho Chi Minh City doesn't appear that great at first, but I loved it by the time I was leaving. Largely just as a place to eat, drink and wander. Nha Trang further up the coast is nice in a way, good beaches, but pretty touristy. I imagine Thailand would be worse though. That's as far north as I got, but apparently Hoi An and Hanoi are pretty great.
I'd say do a loop around south Thailand, and then when you get back to Bangkok, do a loop Siem Reap --> south Vietnam --> north Vietnam --> north Laos --> Bangkok.