One of our favorite cities. We've lived here for 3 months and can share all about it, if you decide to visit. It's a surfing town with beautiful beaches and a calm, laid-back feel. Location is about a 2-hour drive away from San Juan
El Yunque is the only tropical rainforest in the United States. You can hike or arrange an excursion with a group to go cliff jumping and swimming. Search to confirm on whether you'll need a reservation and permit beforehand - the policy is subject to change. Location is a 60-90 minute drive from San Juan.
Vieques is one of the islands of Puerto Rico. It's perimeter is full of gorgeous beaches and wild horses roam the island. It has the world's brighest bioluminescent bay. You'll need a rental car (or rental golf cart) to get around, and it is a short flight or ferry ride from San Juan. Plan at least 2-3 days to make the travel worth it for a proper visit to Vieques.
We've never stayed in Isabela, but we've visited several times and it's a highly-recommended location. There are several great beaches for kids & adults and a biking path along the beach. Some of our favorite beaches nearby are Crash Boat Beach and Playa Jobos Location: About 75-90 minute drive from San Juan.
Far from San Juan, but the southwest corner of Puerto Rico has crystal clear beaches and tends to be drier than most parts of the island. Popular beaches: Playa Buye, Playa Sucia