Golden Gate Park is the environmental center of
San Francisco, California. It is located in the
Sunset area of SF on what used to be an enormous sand dune. In fact the ENTIRE Sunset section of SF used to be a dune but they managed to get grass and trees to grow on it. Pretty cool!
Anyway. There is a TON of cool stuff to do in the park. It is located right next to the SF MUNI N train so it is pretty easy to reach.
It is about 1/2 mile wide and 3 miles long and stretches all the way to the ocean. There are a number of really fun things to do. I would recommend the Japanese Tea Garden, Asian Art Museum and the Botanical Gardens. The Tech Musuem is pretty cool too...
After that more 'touristy' destinations, there are a ton of outdoor activities you should turn your attention to. Basically if you start in the Inner Sunset and walk/bike down to the Ocean you will get the full effect. (You should take your time though)
I Mountain Bike in the park about twice a week and it is always fun. The fact that it is RIGHT in the city and so close to the ocean REALLY make this a special place!