I was born and raised in PR, but now live in Woodridge IL, ORD is now my home airport, and I travel to SJU around 5 times a year to see family. Since you are staying in Condado area, I can also suggest eating at The Place Bar & Grill, it is close to where you are at. Snorkeling in PR is not that accessible, most of the public beaches are not that great for it, the few spots are a bit remote and off the beaten path, with your short time frame, it migth take time away. Culebra is great, but the ferry is a hazzle, if you really want to go to Culebra, it is best to fly there. From where you are, you could fly out via Isla Grande airport, or drive to Ceiba airport. A one day roundtrip can cost around $100 or so, I've actually done it, leave at 8am, and return by 6pm, once in Culebra there is a shared taxi service from airport to beach, around $4 or so per person per trip.
One of my favorite places to hang around is in Pinones,
http://gopuertorico.about.com/od/the.../p/Pinones.htm
There are some remote isolated beaches in there, good local food, I always go to A DONDE OLGA, it is about a minute drive as soon as you pass the bridge. You could do this as a day trip.
Another suggestion I can offer is to stop by a local panaderia, or bakeries. This is where we locally eat breakfast, Kasalta is close by.
http://www.yelp.com/search?cflt=bake...2C+Puerto+Rico
The rainforrest is great, but wear comfortable shoes, and swin suit. Also stop by the local kiosk for food once there,
http://www.puertoricodaytrips.com/luquillo-kiosks/
If you like pork, roast pig, a trip to a lechonera would be good, but they are only open on the weekends.
http://english.zeepuertorico.com/art...k-highway.aspx With the xmas holiday, they will be packed, as families tend to go there a lot during this time.
Enjoy, feel free to PM, too bad we haven't meet before, I could offer other suggestions since we are both locals.