Golden Gate Park is a huge, sprawling oasis on the West side of the city. There are countless things to do and discover including:

• California Academy of Sciences
• De Young Museum
• Disc Golf Course
• Polo Fields Cycling Track
• Archery
• Roller Skating 
• Conservatory Of Flowers
• Japanese Tea Garden
• Aids Memorial Grove (Redwoods) 
• Carousel and many childrens playgrounds
• Dog Park
• Buffalo Paddock
• Boat Lake
• Windmills (x2!)
• Tennis Courts
• Soccer Fields

See more details on the park site here.