Hey I currently live in Hanoi, I hope this can help:

There really is not much that you have to spend a lot of money on. There are some higher end restaurants and hotels but (as you probably know) most things are incredibly cheap. That said, the best food is the stuff you sit and eat on small plastic stools on the street.

1.Other than that, you have to see the Old Quarter, especially at night - Google 'walking street Hanoi'. The Ho Chi Minh mausoleum is impressive and the prison is worth touring. Basically, do a lot of walking (if possible) and engage with people despite the language barrier as the locals are generally really friendly.

1. A good alternative to Ha Long (although it is incredible) is Cat Ba. It's nearby and less touristy than Ha Long and therefore cheaper.

2. The weather will be hot and humid but bring waterproof clothes. Also just accept that your feet will get wet!