There are lots of interesting things to see and do in Puerto Rico, but my favorite in San Juan was the Old Fort. I wish I could have gone through it even more slowly than I did the day I was there. Everything about it was so interesting. So much history there, so many battles fought.
At one point, it was called the Castle San Felipe del Morro, I believe. The views from there are stunning, and the area leading up to it was a great place to fly a kite which my younger son and I did for some time. It is a fascinating place, with neat people. Wish I could go back.