Old San Juan is the historical district of the city (and where we'll be getting married!). Spend some time walking the colorful streets of the old town and you'll come across national landmarks/forts, great cafes/bars, and busy plazas. Check out the link for a popular walking tour option.
If you have an extra day, we highly recommend driving about an hour east for a snorkeling/boat adventure to the island of Culebra. There you can visit Flamenco Beach, which is consistently rated one of the top 20 beaches in the world. We personally prefer the snorkeling guides that provide bottomless sangria and sandwiches...
If you're into rum, consider a visit to Ron del Barrilito for history, tastings, and mixology classes in a beautiful old hacienda — it’s the oldest Puerto Rican rum brand still in production. Casa Bacardí (nearby) is another fun option.