There aren't really fitness centers in Buena Vista--it is mostly a resedential neighorhood. But it borders more active neighborhoods. My fitness center recommendations for a single girl would either to go west to Cole Valley Fitness Center (though it is probably more expensive) or to go over to one of the many fitness centers on Market Street in the Castro: Gold's Gym, 24-Hour fitness. (Don't let the fact they are in the Castro fool you--you will get hit on at these gyms. But if that is okay with you they are better bets.)
