Parque de Bombas, the old firehouse. This structure was built for an agricultural exposition, and later served as a firehouse for a long time. It is said to be one of the most photographed buildings in PR, and you can see why. Inside is a small museum to local fireman.
We also took a tour offered by the museum which took us around the historic part of the city by bus, while a tour guide explained the sights. Our tour guide, Israel, was fantastic! He was friendly, and, even though we were the only people on the tour, he really gave it his all. He waved to every single person we passed, stopping to talk to several, and everyone seemed to know him. He was really fun, and his obvious love for his town was contagious. I highly recommend this tour, particularly since it only cost $2 per person.
Next, the Ponce Museum of Art.