December is SUCH a great time of year to go! You should be able to swim (I'm usually there the first week in December, and even in the evenings it's warm enough for a swim). Heck, if you can get mom to watch your child, you and DH could always get a little private swim in (especially if your hotel has a hot tub!). For the whole family, I'd say that one nice way to spend the evening is heading over to The Boardwalk. If you happen to be in Epcot that day, when the park closes you can walk over (it's a very pretty walk). There are restaurants and places to grab a snack, arcade-style games, shopping venues. Plus, you can go into the Boarwalk hotel and look at the decorations, which are beautiful. Another idea might be to head over to Downtown Disney for some shopping, sightseeing, snacking, etc. Finally, depending on how late they're open during that time of year, while at Downtown Disney you could also head over to Disneyquest (especially if you have the ParkHOpper Plus, or other ticket which includes admission to this HUMONGOUS arcade). It might be pricey if you have to pay for separate admission, but if it's included, it's a fun way to spend some time!
Richard