What part of San Francisco will you be in?
Children's Park (located in Golden Gate Park) is a nice park for kids. I also love Julius Kahn Park located in the Precidio. For fun museum type places, the Bay Area Discovery Museum is a fun place for little ones to explore and learn (in Sausalito). In Oakland, Fairyland is fun that an 18 month old would like, too. In the San Jose area, Happy Hollow Park and Zoo is great fun, as well.
I haven't been to the park lately, as my newborn is only 10 weeks old. But my 3 year old is getting stir crazy lately cooped up in the house, so I need to start getting out again! The weather can be a little fickle in SF, so sometimes I head more toward the South Bay since it tends to be warmer there.
Children's Playground 415.831.2700
Julius Kahn Park 415.292.2004
http://www.nps.gov/prsf/places/jkahn.htm
Bay Area Discovery Museum 415.487.4398
http://www.baykidsmuseum.org/
Happy Hollow Park and Zoo 408.277.3000
http://www.happyhollowparkandzoo.org/index.php3
Fairlyland 510.452.2259
http://fairyland.org/
HTH!