I can’t speak to full itineraries, but I have a few tips, and feel free to ask me specific questions—my friend lived in Vietnam for a year, and we can poll our Vietnamese friends!
That said, I really recommend Sapa. It was my favorite part of the trip, and taking a sleeper bus there from Hanoi went really well. Lots of great hiking, beautiful rice terraces, and a break from the heat!
The other classic destination from Hanoi is Ha Long Bay. If you go, make sure to plan the trip in advance and stay out on the boat! My friend and I just showed up, and we ended up on what my friend called “the sucker boat.” The town is also a bit charmless, so sleeping on a boat as part of a multi-day tour seems superior.
And if I were doing it again, I’d see if I could visit the Hang Son Doong cave … Hang Sơn Đoòng - Wikipedia