I second Hoi An. Even better, I STRONGLY recommend a bike tour from Hue to Hoi An. My friends and I used Le Family Riders. If you're solo, doesn't matter. You can drive, ride with a driver, or share a bike with your friend. They also make sure you know what you're doing before you get going. Absolutely the best thing I've done in all my Asia travels the past four years.
We flew from Hanoi directly to Hue in the morning on Vietnam Airlines. They picked us up and we had breakfast at their house (included) and then a stop at the family coffee shop (included). From 12-7 we all rode with a driver around Hue and hit the sites. You can rent a bike on your own, but with a driver, they know all the sweet spots and tiny dirt roads through tiny villages. Dinner was at one of the legit road side stands for $1 extra.
The next morning at 9a they picked us up for our drive to Hoi An which lasted all day, until 7. You'll find many blogs on the trip. The scenery was amazing and the drive up and over the mountain pass was gorgeous. It was fun to stop at the swimming hole and for real, the seafood lunch included was GREAT. In that long day tour, everything is included - every water, beer, coffee, ice cream, snack....and you can get as much as you want without any hesitation. They also feed you well. Grand total is like 50$ for that long day drive.
Hanoi is just there to get you to Halong Bay. It's also QUIET after 10 PM. You can easily enjoy a few days in Saigon.