Opened in 1895 with a hyper-modern rebuild in 2005, de Young is among the oldest and best museums in the city - inside and out!
The observation tower is on 9th floor and doesn't require admission. Also free-for-everyone are the outdoor exhibits, including Andy Goldsworthy's sculpture right through the entrance.
The popular Cal Academy of Science sits across the courtyard for those doubling-down on museums :)
Address: 50 Hagiwara Tea Garden Drive, SF, CA
Website: http://deyoung.famsf.org/deyoung/events/calendar
Wikipedia: http://en.wikipedia.org/wiki/Deyoung_museum