I've been to Vietnam three times but the last was in 1986, so I don't know how helpful I might be.
Halong Bay is a must-see. You will drive part of the way by car and you will have to go on at least two ferries. It's friggin' out of the way. I think you get on the first ferry in Haiphong, which was heavily targetted by US bombers during the war.
The hotel I stayed in was crap. It was an old French thing where the infrastructure had declined and was not maintained. We didn't even have running water. I hope the hotels have improved since then, and I'm sure they have.
In Hanoi, visit Ho Chi Minh's tomb and his "house and grounds." There are a few museums there as well. You can see the memorial of McCain which stands by the lake where he fighter/bomber plane was shot down.
In the South, I liked going to Bien Hoa, where the US used to have a base. You can visit the towns along the Mekhong Delta. I never went to Dalat, but it's supposed to be a pretty place.
Hue would be awesome to see. Somehow I missed going there during my three trips.
Saigon is fascinating, much more so than Hanoi. I'd spend some serious time there.
Hope that helps!