Pleasant Hill Park at 147 Gregory Lane, Pleasant Hill, California.

A multiple purpose park, with a 2-5 year old fenced in play area. The sand area has a lot of redwood leaves and twigs, so I recommend leaving shoes on. But the Sand crane and Sand climbing structure are awesome.
The big Kid area does not have 2-chain swings, but does have a couple of tire swings and the play structure is extensive. It's a great park and a good place to let them run around. However there are a wide range of people, so you do have to pay attention.