If you like nature, hiking and seeing the splenders of a Rain Forest then El yunque is a nice place to go to, its a small Rain Forest, but beautiful.
If your going to the Rain Forest, I recommend staying at the Casa Cubuy Ecolodge http://www.casacubuy.com, its a friendly place inside the Rain Forest.
Also Recommend: El Jibarito Restaurant. Mostly locals go there but you might see some tourists as well. They give you so much food for a good price, all kinds of local dishes, its a great treat just outside of the City.
Finally, If you go into Old San Juan, its better to park the car and take a nice walk around the area, its not a big place, and you get so much out of walking there then driving. The Copplestone Roads and 500 hundred year old buildings are so much better to look at walking. I like going outside of the City as well, if you get a chance, its worth doing.over 6 years ago