The drive between HB and Hanoi is not very interesting. You would spend half your time getting to an

Though cruise line suggests Hanoi is an option, it appears that getting there can take 3 1/2 hr each way from docking area! We have only from 11am to 1:30 am in the morning! (13 hr) What do you suggest to make our time meaningful and memorable?
From where the junks dock in HB, it is a good three hours to Hanoi, could be even longer. Not sure what you would do hanging around Halong Bay - I assume the cruise you came in on went past the karst formations which are stunning. free pakistan travel brochure If not, a jaunt out into the bay to get looks at the formations would make a nice afternoon. Also depending upon the weather you could do some kayaking if that appeals to you.
Leave leave from Singapore (2 full days to visit) w/ stops in Ko Samui, Thailand, free pakistan travel brochure Laechobang (port for Bankok- overnight), Saigon(Phu My), Nha Trang, Chang May (port for Hue and Danang), Halong Bay and Hong Kong.
If you have enough time to do an overnight in Hanoi, then I would go there. You can spend time just wandering the streets of the old city, soaking up the atmosphere, sampling some food, wandering around the lake which is close by the old city and the French quarters. Hanoi is fantastic.
The drive between HB and Hanoi is not very interesting. You would spend half your time getting to and from Hanoi, in order to spend, say, six hours in a town which is easily worth six days. You could hit some of the high points listed in all the guidebooks, but I suspect you'd miss the charm.