Catch the BART out to Lake Merritt in East Bay, wonder around and head to one of the many great bars / restaurants on 19th / Telegraph.
Go to North Beach, check out Spectacles + City Lights bookstore. Grab a pizza at Tonys (sit at the bar), then walk it off up to Coit Tower.
Bart to 24th in the Mission, wander down 24th St and grab a coffee at Haus on corner of Folsom. Find a taco joint for $1 tacos. Climb up Bernal Heights then drop down into Courtland Avenue for a beer.