The lake is a great place to take your dog. They can run loose and even swim (warning: a viewer recently told me this is now subject to a citation). Not recommended after dark. There are also other trails at the top as well as the Jerry Garcia Amphitheater where Jerry Day (see next paragraph), Free Shakespeare at the Park and other events are played. See map below.
The big event of the year at the amphitheater is the Jerry Day Festival held every year in honor of Jerry Garcia here in the Excelsior neighborhood where he grew up. Deadheads love it! For Jerry Day 2017 the show will (tenatively) start at 11:15 am on Sunday, August 6, 2017. The show is free but (big) donors can get reserved seats. See website and yelp.