The answer to your question depends on your tolerance for weather and your willingness to pack for it. December could be 55F here. Last year though, we had a week in early December that hovered right around 0F. Snow is rare in December, but it could happen. It could also rain.

I don't know of the CAF cruises are still open, but I don't think they'd be much fun in December. It can get cold and windy on the river.

If none of that scares you away, then you'll be fine. In reality, nothing in the city ever closes. Maybe once every 5 years, there's a city snow day where public buildings like museums are closed. Even then, you'll find many restaurants, bars, etc. still open. On those days, we put on our snow boots and walk to the local tavern for dinner and a pint.

And, by the way, there is almost a 0% chance of having a snow day like that in December. I'd expect normal schedules for everything, as well as 100% of public transportation in operation.