If you want to hang with the locals in the street, this is the place to be. El Mercado de Santurce turns into an all-out street fest at night on the weekends. Bars lining the square flow into the street and two stages (one with Spanish Rock, one with salsa) play for the crowds. Some bars also have live music. San Juaneros of all ages dance and party in the streets. Beer flows. Menus are generally only in Spanish. There is free "entry."
If you are comfortable with the fact that you may stick out like a sore thumb (i.e. not being Puerto Rican), and you want to experience a local event, this is for you. Many people may want to talk to you and practice their English, see how your trip is going, and flirt. If you are uncomfortable with this and with all eyes being on you, this may not be an ideal destination for you.
Beer service can stop anywhere from 11pm till 2am, all depending on the crowd and the mood.