Average high/low for December is 76/60. That's a little chilly for the pool or the beach IMO. Cycling over on the Palm Beach trail is fun. MacArthur Park is great for a day. Check when they have evening concerts. Thursday night at Clematis St. is usually a good time. Plan a visit to the Fogler Museum and a little time at The Breakers hotel. For about $350 you can rent a boat at Juno and tour the inland waterway. Some think that's expensive but only those who have never owned a boat.
After you've done all that buy a good bottle of wine and sit on the balcony and enjoy the view.