Visit Old San Juan but don’t spend too much time there.. it’s the place tourists never leave. Get a car rental and make a big circle around the island.

Visit Cabo Rojo/Playa Sucia in the southwest, beautiful cliff views (was in a scene in the movie “Perfect Getaway”) and lighthouse.

Take a half-day ferry trip/picnic to Gilligan’s Island.

Visit Ponce and the marina there and just walk around the town.

Do stop at the little cook-stands at the side of the road and buy fruits or pinchos (kebobs) or other meals.. don’t be afraid.

Don’t do anything at night, most places are closed anyway.

If you head into the middle of the island, there’s some really hilly (cartoon-y hills almost, because it’s like riding hills that look like Us) Fun to see whole towns built onto these steep hills.. it’s a crazy but fun drive.

Visit El Yunque, the rainforest (My husband proposed to me there after a long hike :) )

If you can get off the island for a night, take the $2.50pp ferry ride to Vieques or Culebra (wild horses, secluded beaches, best bioluminescent bay nighttime kayaking ever in vieques <- lifetime must do).

That should be enough to do in one trip. Enjoy!

ps. my profile photo is me on a reef at Novia Beach in Vieques

DramaQ basically nailed it. I would add one thing: no matter where you are staying go on a Thursday, Friday or Saturday night to the El San Juan hotel. Dress up and be prepared to salsa the night away. It is Ground Zero of social night life in Puerto Rico! Stop at the Oval Bar in the main lobby and have James, the head bartender, serve you up a couple Mojitos. Take a cab and just say “the El San Juan.” Everyone knows.

If you go to El Yunque, be ready to swim. One of the trails has a waterhole along it.

For the rest of the island, don’t be afraid to get off the beaten trail. My wife and I found a nearly deserted beach on one corner of the island.

If you’re comfortable speaking Spanish, there aren’t many places in the island I’d warn you away from. Like the US, if a neighborhood looks dodgy, stay away. Most cities have something worth looking at, though.

If you’re party people, definitely spend some time in “New” San Juan, in the Isla Verde region (where are the hotels are). There are lots of great clubs. But Old San Juan is the heart of the island in many ways, with it’s old-fashioned architecture and the fortresses – El Morro and San Cristobal. Spend a day there, but don’t stay for your whole trip.

If you want to branch out to other cities, my first recommendation is Ponce. They have a world class art museum (if you’re into that) and a lot of old world charm.

El Yunque, the tropical rain forest, is another great place to go if you’re into nature/hiking/etc. AS sipes23 said, be prepared to swim.

Another place I love is in San Germain; it’s a tiny run down town, but it has the remains of the first Christian church built in the western hemisphere.

If you want to explore the smaller islands, Culebra is amazing.

I’m partial to the northern part of Puerto Rico, since that’s where my mother’s family is from, but I’ve found something wonderful in every corner. (which reminds me – in the south there’s a place called Parguera, which has a phosphorescent bay!)

I’d recommend flipping through a guidebook and seeing why interests you -especially in the cities – so you can weigh your options.