I'd say 5-7 days, for the reasons given by the thumbs-up reports above. Also, try sea kayaking -- we did this with my kids when they were in their teens, and for once they were awe-struck... a pod (school? flock? hoard?) of dolphins came up to them and wanted to play!

Buy a local map, BTW...you'll want to be able to explore away from the main drag.

By December, when we've gone, the surf is generally too strong on the north shore for kayaking and snorkeling - east and south are better. As for hotels, I'd stick to the south side for those reasons. There is a Marriott near the airport which is a good point for going to either the east/north or south/west...but that harbor gets cruise ships which can be annoying.