Golden Gate Park is a VERY large park on the west side of San Francisco. The contains the Conservatory of Flowers and the de Young museum. The California Academy of Sciences is scheduled to reopen in 2008. You can easily spend a day here, especially if you walk around the whole park, which if you are a decent walker will still probably take three or more hours.
Parking is available at 10th and Fulton for $2.50 an hour (as of this writing) however, there are spots available along the streets in the park for free. The further west you go, the easier it will be to find a free spot. The park is closed to cars on Sundays, making biking and roller blading on those days awesome.
The de Young museum is currently the #1 reason to visit the park. In fact, if you don't want to see this museum or the Conservatory of Flowers, you can probably safely skip visiting the park and not miss anything you can't see elsewhere. It looks ..odd.. from the outside, but it is a nice museum and the outside look is really growing on me, it is supposed to look even better as the copper outside turns green with age.
The Japanese Tea Garden is a nicely landscaped garden with sculptures, waterfalls and streams. Unfortunately it is very busy. Entrance costs $3, but you can get in free M, W, F mornings.
The Conservatory of Flowers, at the east end of the park, is your typical botanical garden, although it is older than the rest. At $5 it is a great bargain compared to the rest of San Francisco.
At Stow Lake, the largest lake in the park, you can rent bikes, kayaks, or row boats. It is pleasant to slowly make your way around the scenic lake dodging ducks, gulls, and other inept rowers. Apparently even dogs are allowed!
The park also has large grassy fields to do standard park things on.
At the very west end of the park is Ocean Beach, a great place to hang out on the sand and people watch. Do not enter the water here however, very strong currents can sweep you away- just three steps into the water.