The first Tuesday of every month, both the de Young Museum in Golden Gate Park and the Legion of Honor in Lincoln Park waive general admission — no tickets, no registration, just show up. Hours are 9:30am–5:15pm at both locations. Neither has an age restriction for the free day. BART to the de Young: take the N-Judah or 44 O'Shaughnessy bus; there's no BART stop that drops you at the door. For Legion of Honor, the 18 46th Avenue bus runs from the Richmond.

These are the Fine Arts Museums of San Francisco's two flagship buildings, and the permanent collections alone justify the trip — the de Young holds American art, textiles, and the knockout African and Oceanic galleries; the Legion runs European paintings through Rodin sculpture. First Tuesdays tend to draw families and retirees in the morning, younger crowds after noon. Neither museum is a ghost town on free days, but they're not unmanageable either.

Practical note: the de Young has a paid underground garage off Concourse Drive, but Golden Gate Park also has free street parking on JFK if you're early. At Legion of Honor, street parking around Lincoln Park Golf Course is usually findable on a weekday.

If you have two hours, pick one and commit. The de Young's textile and costume collection on the lower level is chronically undervisited and worth the whole trip on its own.