F_M got the most of it.
For a street food extravaganza, go to Piñones (it is only 10 minutes away from the airport). It is better on weekends, but you can go there any day or time and find several places open.
Food you must have if you want to claim you tried Puerto Rican food: Alcapurrias (they come with all different fillings, you should try jueyes, a local land crab). Piononos, Bacalaitos, Whole Fried Red Snapper, Lechon Asado. Pasteles are also a big item here, but they are not as easy to find. We are close to Christmas, so you might get lucky as this is the season for them. Mofongo is the flagship of the local cuisine. Order it stuffed with whatever fancies you. You will see Pinchos everywhere. Feel free to skip them. They are nothing out of this world.
Las Cuevas de Camuy (the caves) are right next to the Radiotelescope at Arecibo. It would be kinda silly to go to one and miss the other. At the entrance to the caves, they will hand you a flier for a nearby restaurant that is actually pretty good (I forget the name)
Definitely head west. That’s where the best beaches are. If you really hate your skin, go to the beach right next to the Lighthouse at Cabo Rojo (Playa Sucia, I think is the name). Beautiful spot.
There are also some good beaches in Isabela. You can go see El Ojo de Jacinto (a natural bridge and blowhole on a rock promontory, right next to the excellent beaches of Villas del Mar Hau.
At the risk of this post being intercepted and getting myself thrown out of here, Puerto Rican rum sucks. Sorry. Try before you buy. A surprising find, though, is Anis Paloma. Even if you don’t like Anise (does anyone?), you have to try this stuff. Incredibly smooth. They serve it mixed with 7-up (I think the name for that is Chichaito, which is kind of a dirty word, btw), but I reccommend you try it alone as a shooter.
I will reccomend El Ynque if you hail from the frozen north. If you have ever been to any other tropical rainforest, think nothing of skipping it. Unless you are into hiking, then go. If you do, wear a swimsuit. There is a couple of falls that are good for swimming.
If you are going to the place I figure you are going in Dorado, you will like it. It is a nice place.
Oh, and of course, you must schedule a day to visit Old San Juan. Weekends are best. IF you go on a Sunday, I will be glad to join you over there with my family. We have some bitchin’ kites that I can talk my boy into sharing. Expect a beer on the house. Otherwise, you can buy regular kites for $3 right on the lawn to the Fort (a must visit, btw). Don’t miss the chance to fly one there, they literally jump off your hand.
Now that I am subscribed to this thread, fire at will with any more questions you might have. I have been receiving tons of guests recently and I am mostly current in the touristy scene.
Are you all about beaches? Drving some segments of the panoramic road through the mountains can be a real treat if you time it right. Let me know if you care about this and I will advise where and when to go.
Are you willing to sleep out of your prearranged hotel? That could make it possible to add some more distant destinations. Drive times here can be deceiving. Although you could easily drive around the island in one day, that doesn’t mean it will be fun. Do not estimate 60mph when computing travel times. 30mph is more like it.