Some things to consider. If the weather is good a day at the beach would be cool. Black's Beach is famous and has a nude and gay section. It is very hard to get to tho. Decent hike down (and then back up) some steep cliffs.
Sometimes I like ot go near the Hotel del Coranado to layout. The navy seals train there and more often than not, a platoon will run by shirtless.
Balboa Park is great. BIg gathering spot on Sat and Sun for volleyball. The Zoo is at the park and is truly one of the best in the world. Also, lots of muesums there.
For nightlife check out Gaylesbiantimes.com