Three days in Hanoi was plenty for me, and the Intercontinental Hanoi West Lake was very nice (and they actually recognized Spire members).

I'd take the extra 2 days and go to Ha Long Bay because it's absolutely beautiful ... but if you're a "night life" type of person, book one of the overnight junks that caters toward the party crowd.

You can read about our time in Hanoi and Ha Long Bay on my blog if you like. It was done partly independently and partly as part of an organized tour.