Cool State Park. It's the place where gold was first discovered in California, starting the gold rush. There is a good museum with real gold, a small ghost town, a lumber mill, and gold panning for the kids. If it's a hot day mining pans are for sale if you and the kids want to pan for gold in the American river which passes through Coloma.
My family and I had such a fun time travelling the state route 49 to the Marshall Gold Discovery State Historic Park. The drive getting there is beautiful. And if you love history, you will love this park. This is the place that started the gold rush! We were able to see beams from the original Sutters mill! Our kids were able to pan some gold right there in the American river. We didn't find out retirement nugget, but playing the water is always good fun. If you are a history buff, or just love to consider all of the millions of dollars flowing through the American river, you have to visit this state park.
We visited the Marshall Gold Discovery Park several years ago and we had a blast! Everything is right there, from the location of where the gold was found, to the mills, to the little stores. We loved it. We looked for gold. You can go to the river for free or pay $5 and get to be shown how to do it and you are sure to find a couple of little nuggets. We looked like little kids having fun!
Kids will love panning for fake gold in front of the park store. You can also pan for real gold in the river, but I don't think there is much there.