I don't think I would want to visit Hanoi without seeing Halong Bay, for most people it's the highlight of a Hanoi visit. Though I'm sure as you say there is plenty to keep you occupied during the 3 1/2 days you will be there.
I don't have any recommendations for the capital as I've only visited the south of Vietnam. I've been to one or two places where I didn't see most of what was on offer and told myself I can always return, Vietnam being one of those places. I thought I would one day visit the north and centre of the country plus take the Victoria train to Sapa in the mountains near the border with China. It never happened. One or two other places I thought I would return I now wished I had taken the opportunity to see all what was available at the time.
Maybe Halong Bay isn't all it's made out to be, but even with the chance of a cancellation I would be tempted to book a trip, what's a few hours extra travelling when you have luxury of recovering at the Centara to look forward to.
The hotel in Hanoi looks lovely Fiona.