There is so much to do! Here are just some suggestions:
Walk around Old San Juan
Explore the various other neighborhoods and beaches of San Juan, such as Condado or Ocean Park
Visit forts El Morro and San Cristobal
Hike El Yunque & visit Luquillo for lunch
Tour the Bacardi Rum Factory
Take a day trip to the island of Vieques
If you can, rent a car and explore outside San Juan!
Check out Viator for endless options