If you have kids and are going to San Francisco, you really should plan to spend some time at the Exploratorium. I currently am staying about 4 blocks from the Exploratorium, so I jog or walk by it almost every day. It is in the Palace of Fine Arts, which is a structure built in 1893 in a Roman/Greek architectural style as part of a world exposition held in San Francisco. It is surrounded by a fantastic park and sits right on the Marina Boulevard waterfront with a nearby beach, yacht club, in view of Golden Gate Bridge and within walking distance to Ghiradelli Square, Fort Mason, The Beach and Hyde cable car terminal, and other downtown attractions.
The Exploratorium is one of the best interactive educational museums around. Every first Wednesday of the month is a free admission day. But it's worth the price of admission if you can't cash in on the free day.