California Academy of Sciences
A 400,000 square foot world-class scientific institution in Golden Gate Park which includes an aquarium, a planetarium, a natural history museum and a four-story rain forest with a living roof.
When you go in first go get your separate but free tickets for the planetarium tickets (behind the planetarium … ask someone where). Then check out the rain forest. These are the two busiest, and best, things to see.
There are many other special programs and permanent collections as well. For example, the Academy has 40,000 live animals, an entymology (bug) collection which is one of the four largest in the U.S. and a fish collection that includes more than a third of all fish species known to science.
Free weekends for residents only
Free weekends are usually crowded so get there early. Hours are 9:30 am-5 pm weekdays and 11 am-5 pm Sundays but confirm details at calacademy.org/admission since they sometimes change. This page also contains current pricing and various deals which are well worth checking since a regular adult admission is $35.95 and climbing.
It appears that free Sundays for everyone program has been cancelled. However, free admission on certain dates is still available to San Francisco residents only (proof of residency is required). See details.
Another good way to see the museum for only $12 is to attend a Nightlife event, every Thursday night from 6-10 pm. These are adult only (21+) events and an ID is required. Cocktails and sometimes dancing are available. See next dates & details.