Who's been to Puerto Rico?S

Just booked a much-needed, four-night stay at the Ritz-Carlton San Juan this spring! I'm pretty psyched. I've never been to Puerto Rico before, and it looks beautiful (at least the tourist-friendly parts are). Who's been before, and what are the can't-miss things to do/see in PR? I'm told I need to see the historic Spanish forts, but that's the only tip I have so far.