Pacific Beach is a great place for swimming, surfing, and bodyboarding. There are a good number of bars, restaurants, and places to stay.
Check out the yearly Pacific Beach festival: http://www.pacificbeachfest.org/pbfest/index.htm
Or, the largest annual Brazilian festival on the West coast: http://www.braziliandaysandiego.com/
Just walk around to find a number of inexpensive beach restaurants, featuring mostly Mexican or California fusion.
Rent a bike to cruise the area: http://pacificbeachbikes.com/
Or, take a surf lesson or rent surfing equipment: http://www.pbsurfshop.com/
Tip: It definitely gets tight with traffic here. I recommend coming early for parking, or taking a cab and spending all day here playing in the surf or walking the boardwalk for people watching!