Certain areas of San Juan are noted for their outdoor and street activity after the sun sets.  These areas have a high concentration of bars (most are open-air) and are excellent for people-watching.  Some of the areas prohibit drinking on the street, but you can always watch the street action from the comfort of a bar stool.  These areas generally don't get busy until after 11.  Often, the party continues until two or three in the morning.  Also, these areas will frequently host block parties or street fairs.  On those nights expect a lot of exciting activity. Cross reference the Calendar of Events for special occasions.