La Han Bay
 

Better to do 2 nights or one night cruise? Have only 2 1/2 days in Hanoi if we do 2 nights for Halong bay cruise. Otherwise could have 3 1/2 days in Hanoi with a one night cruise. Sapa our other destinations for a couple of days in northern Vietnam

In my opinion Hanoi is infinitely more interesting than Halong Bay which can be a traffic jam of boats. I recommend not doing it at all & if you do you'll know immediately after the long uncomfortable drive why, knowing you have to do it again to return. Watch 'Indochine' instead to get a real feel for what the bay once was & spend your time in one of my very favorite cities.

To an extent, I agree with MmePerdu. Halong Bay is overhyped and overcrowded but you are talking about Lan Ha Bay its smaller quieter neighbour. Though if you haven't been to Hanoi before, I would skip any bay at all. Hanoi is a fascinating city that I never tire of and have lost count of the number of times I have visited. The last time was a few weeks ago on the way back to London from visiting family in Sydney. It deserves at least five days to do it justice. I feel much the same way about Sapa which is vastly over touristed to the extent that it is like a theme park in places. The scenery is undoubtedly impressive but unless you are prepared to get way off the usual tourist tracks, it wouldn't be for me. That said , it may be quieter now the hordes of Chinese tour groups are kept at home! Two days is not long when you consider it will take a 9 hour overnight train or 5-6 hour bus ride on the new highway to get there.
There are much nicer, equally impressive places to visit in the north that are easier to get to. If you did want to spend a day outside of Hanoi, I would have think about Ninh Binh, often described as Ha Long on land an overnight there would make a nice trip from Hanoi some photo etc on our blog @ https://accidentalnomads.com/2014/12...d-kingfishers/

As an alternative to Sapa, parts of the Mai Chau Valley would be nice (not in the main village and not at a weekend. It is closer, around 2-3 hours from Hanoi.