Love attending music festivals. Been to Bonnaroo, Coachella, and Lollapalooza, as well as smaller venues like Pitchfork and FYF Fest. They all have unique merits and faults -- especially the big festivals -- but hey, live music. Lots of great, funny stories.
Film festivals, too. I try to break away from school for a week/weekend to attend one, usually as an intern. Worked at Cannes earlier this May and it was wild. My college town hosts Ebertfest and that's always fun to attend.
Love more communal things as well; every last Friday of the month, Chicago bikers gather downtown and follow a main path through the city. A large park also hosts free movie nights and music performances; sure other cities do this, too.
Sports events! Wrigley Field next Monday for the Cubs first home postseason game.