Traveling to Vietnam Advice.

Hey everyone my partner and I are about to buy a plane ticket to Vietnam. We will hopefully be going in the spring and exploring Northen Vietnam for 2 weeks. I've never been to Asia and ive had some difficulty finding reliable answers to some of these questions I'm about to ask. First I want to travel to three places (I'm open to ideas here) First explore Sapa valley. I have no idea how to get there or where to stay though, I saw Sappa Sisters might be good? Only issue I would be up for hiking all day but I think my partner could max do 5-7k (3-4 miles I think) a day. I'm worried Sappa might not be right for us. GF is a little on the heaver side and I'm worried it would be hard for her. Buuut I really want to go is this possible? Where do I go online to start the process to get to sappa probably FROM Hanoi flying in. Second, I want to go from sappa maybe to a Ha Long Bay cruise? I really don't want to break the bank but I want to a luxury cruise experience one with great food, good experiences, and good food cooking classes if possible, and cultural classes too maybe? Third I want to explore Hanoi before we head out. Does anyone have great affordable accommodation requests in here? I think food tours and temple tours might be our thing here. Any reccomendations you have for any of these cities PLEASE let me know. I'm so excited for this trip, I just want to do it right!

Budget is estimated to he around: $3000-3500