DeYoung Museum and free Tower

Diverse museum ranging from regional ethnographic art (Mexico, Africa, Papua, etc.) to European and North American fine and modern art. Founded in 1895, the current building opened in 2005.
Free entry first Tuesdays 10am-5pm every month,
and Fridays 5-8:45pm from April -November combined with free events: deyoung.famsf.org/programs/fridays
Always free to take elevator to observation tower for fantastic views of the whole Golden Gate Park and City of San Francisco. deyoung.famsf.org

The museum was established in 1895 as an outsourcing of the 1894 California Midwinter International Exposition. After extensive renovation work in 1949 and severe damage in 1989 during the Loma Prieta earthquake, it has its present appearance after a reopening in 2005. The building was designed by the Swiss architects Herzog & de Meuron.
Source: Wikipedia

Enter the museum take the free elevator up the tower to get a nice overlook from SF