Halong Bay is a 2-day trip with overnight cruise. The ride from Hanoi to Halong takes at least 3 hours one way so you should not try a day trip.
Hoi An is a quaint, little town with great food and lots of historic sights. You can probably cover the whole town comfortably in 2 days. The flight from Hanoi to Danang takes about an hour and it takes about 30 minutes from Danang to reach Hoi An.
I did a week-long trip starting in Hanoi -> [flight] Hue -> drive to Hoi An -> Danang -> [flight] Hanoi -> Halong Bay
Keep in mind the weather in Hoi An is quite different than in Hanoi - the rainy seasons are switched so you will likely get sun in one and nonstop rain/drizzle in the other.