I've been taking solo trips to Vietnam since 2008 - total of 6 trips with one coming up in March. I also like the very same things - esp. the food and people watching parts. Both Hanoi and Hoi An should be on your list, but they couldn't be more different. You need to do considerably more research B4 the trip, start here at
Travelfish.
First of all, fly a good carrier to Asia - I like Asiana because they treat economy passengers very well, and seating is quite spacious (1 connection in Seoul). If you'd like to PM me for detailed info on hotels, I do know a few really good ones (I prefer small boutique-type 2 and 3 star places).
I took a cruise to Halong Bay earlier this year, and would never do it again. We're all different, but it did nothing for me, and considering the cost for a decent boat, I consider it a poor value in a country of true values.
You don't want to rush around in Vietnam trying to see too much - the really big mistake that tourists make here. Domestic air travel is the way to get around since road travel is often dangerous, and trains are ok, but slow and the service is limited to just a few areas. My favorite places are these: Hanoi, Sapa, Dalat, Quy Nhon and Hoi An. I will only be in Hanoi and Hoi An on my upcoming March trip since I'll also be in Malaysia. It's a wonderful and friendly country, and they really do love Americans now (also a great place to travel solo and meet like-minded people from around the world).
I don't recommend travel to the north of Vietnam in Jan-Feb for 2 reasons - it's much cooler there at that time, and the Tet holiday should be avoid by first-time travelers. March is a great time to go - my favorite.
Do you actually mean 3 days for Vietnam in total? You're kidding, yes? If that is so, I'd pick another country - it's simply not worth the great efforts needed to travel to VN. Come back when you have at least 3 weeks - this place is awesome and really deserves that as a minimum, I kid you not.