Adventure Playground at 160 University Avenue, Berkeley, California.

My kids' favorite place in Berkeley. They could spend good 3-4 hours easily here. They keep themselves very busy with building forts with boards and also hammering away. Great way to be creative with building materials. A little bit of supervising for safety (using nails and hammers), it's a great place for kids to experiment.
The zipline there is an icing on the cake!
All the staffs & volunteers I've met are so friendly and dedicated. I've noticed that they were always very patient with kids.
This is one of those places that I would recommend to visit and experience while visiting SF bay area with kids in age of 6 to 13. It's now much harder to find this sort of kids playground in US.