I agree with the previous post that 5 days in Hanoi is too much. As someone else mentioned you could do a day or two in Hanoi and then an overnight cruise on Ha Long Bay, or visit Hội An. I have also heard great things about Da Nang, but did not visit on this trip.

We seem to have the same interests . . . my girlfriend and I were more interested in the food and culture then the museums. We used, and I would highly suggest, a student tour guide (hanoi kids). Our guide took us to hole in the wall restaurants throughout the city and provided us an insight into her daily life and culture. If you do Hanoi or Saigon, I highly highly recommend trying to book one of these student guides.

In terms of weather, we just visited at the end of June 2015 and it was HOT! It reached 110 fahrenheit one day and was in the high 90s at night. We were sweating after a couple of blocks.

If you have status with marriott, I would recommend a night at the JW Marriott Hanoi. They treated us very nicely as golds and had an excellent lounge. It is a bit far from the city (30 minutes by taxi). We only stayed on our last night before flying out.

As others have mentioned, Bali is amazing and I would probably choose it over Hanoi for a honeymoon. In fact, I am in the process of planning a destination wedding for this December in the northeastern part of the island. Nusa Dua is going to be your touristy area, a compound of world class hotels. I would avoid Kuta/Legein, and either stay in Sanur (laid back beach town) or Jimbaran Bay. You could easily split up the beach time with a couple of days in Ubud.

Good luck!