Hi enrique, and Welcome to Cruise-Chat!
The beaches are ok, not the best in the Caribbean though. The Condado beach and hotel area is a short taxi or public bus ride over from the pier. I've taken the bus over a few times, and a taxi once. I think the taxi (at the time) was around $12, and the bus 75 cents. If you take the bus, the station is a short walk from the pier. Exact change is required, and when you catch the bus back you need to be at a "Bus Stop/Parada" sign and wave. I think it ran every 30 minutes or so.
Personally, I just like to walk around Old San Juan and soak up the culture and history of this great island. I've found the people to be very friendly. On the next visit I'm going to go on the free trolley that Mike52 mentioned.
El Yunque, the Rain Forest is also an awesome tour, though is a bit time consuming. The Bacardi Rum distillery tour is always nice.