On a budget? Many towns in Puerto Rico have municipal trolleys that go around the city center. These are not guided tours, simply free (yes, free) transportation and a way to get an overview of the city or to reach a point of interest more easily. Aside from the ones mentioned here, several cities including Ponce, Guaynabo, Bayamón, Carolina, Cabo Rojo, and others also have a public trolley systems.
The town of Caguas is about 25 minutes south of San Juan and also offers free trolleys that go around the city from Monday to Friday, from 7:30 a.m. to 3:30 p.m. Currently the three stops are: the public bus terminal; Plaza Santiago R. Palmer, which is the main square of Caguas; and Plaza Gautier Benítez, a shopping district. If you disembark at Plaza Palmer, visit the Caguas Tourism Office for a self-guided walking tour of the municipality's free museums or just enjoy the beautiful square.
The town of Mayagüez on the west side of the island has 9 trolley routes, of which three stay within the city, while the other six go out into the countryside and adjacent towns. The main routes pass by historic monuments, the Cathedral and City Hall, the Farmer's Market, the brewery Cervecera de Puerto Rico (where Medalla and Magna beers are produced), and several museums. The trolleys run every day from 6:00 a.m. to 6:00 p.m.