SFO is one of the better U.S. airports for leaving on a layover because storage and rail both exist. The trick is resisting a full San Francisco itinerary.
What to do with bags
SFO says baggage storage is available at the Airport Travel Agency on the International Terminal departures level. Items are x-rayed before storage, and rates vary by item size.
Leaving the airport
BART works for the Mission, Embarcadero, or a focused downtown meal. Rideshare can work for nearby waterfront or hotel plans, but traffic should decide the final choice.
Getting back through security
SFO notes passengers can walk to all gates once through a checkpoint, but you still need time for storage pickup, AirTrain if needed, security, and boarding.