We'll be in Hanoi for 5 days from the 9th of March.
We'd like to do something from there (I don't like cities much)
What can we do from here?
Cat Ba? (Speed Ferry from Halong city + one day halong bay + maybe some motorbiking around Cat Ba)
Sapa? (Do we have enough time?)
I'm leaning towards Halong bay, because of time. Can we fit Sapa? I'm worried about Halong Bay's dodgy tours..
We'll also spend 5 days in Hoi An (from where we'll do a 2/3 day motorbike tour)
Sapa 's great but I wouldn't recommend going for less than 3 days / 4 nights unless you sleep well on trains. Still, in 5 days you'd have time to do that and get a glimpse of Hanoi too, but couldn't also do Halong.
If you decide you prefer Halong Bay (there's a thread on here somewhere with people debating the merits of Sapa vs Halong Bay, which might be worth a read) then as you've noted, you have two options: a tour or self-organised. Choice really depends on whether you're happy to sort stuff out yourself or want it nice and easy. Don't be put off by 'dodgy' tours - go to a recommended agent or pay for one of the more luxury boats. It might cost a bit more but is worth it. I agree with daawgon about Cat Ba town too! But think Hanoi has a lot more going for it than Old Quarter.