I'm going to Puerto Rico in July what are good family activities to do while down there?

I am going to Puerto Rico in July for 2 weeks with my Dad (who's 53) and my twin girls (who are 3) for vacation. This is our first vacation that we have ever taken together. We are renting a car and staying in a hotel in San Juan. He has already made all the reservations and paid for the hotel and the plane ect; however, he has given me the task of figuring out family friendly activities to do while we are down there. All I can think of is, well...the beach! and El Morro. Any other suggestions!??! I do have umbrella strollers! And I'm not afraid to walk! This is my first vacation since I've had the girls and is way over due! (last one was in 2006). I don't mind going out of San Juan either. Before I had the kids I USED to be a bit of a traveler and go to places like Germany, Japan and South Korea just to name a few places. So I'm game for anything; however, with my dad being in his 50's and the girls being 3, I don't know how well they would fair. So any suggestions that would be insightful would be greatly appreciated. Thank you :)