Rancho Seco is a good place to fish, raft/canoe or take a float tube or small boat (electric motor only), swim, and camp. In the summer, you can catch a lot of bass early in the morning before the sun hits the water on floating rapalas and later in the day around the many weedlines lining the lake with senkos or split shotted roboworms. It is a good place to fish with kids and is really clear water. If they bring goggles or a snorkel mask they can see bass and sunfish in the swimming area.
It also doesn't hurt that there are some GIANT bass in the lake that I occassionally hear about people catching. I had about an 8 pounder follow my spinnerbait in once and know of at least two fish over 10 lbs caught there.
Take your kids, or don't, and have a good time! It is within an hour's drive from Folsom.
Chris