While Mount Diablo State Park is big and expansive, especially for little legs, Rock City is small and confining. I personally like exploring big wide spaces, but these kids of ours sure loved Rock City. Crevices to explore, rocks to climb, holes to squeeze through. It's like a big jungle gym without the swings.
There is more to it, of course. These exposed sandstone rocks have stories to tell. There's a geology lesson here. There are vistas and excellent picnic spots. There's a map to navigate and signs to follow (or not).
You don't have to walk far to get to Rock City from the parking lot. And you won't find yourself traveling far beyond it. So this isn't a 'let's go stretch our legs' kind of an outdoor adventure. It's an adventure of balance, strength, coordination, risk-taking, and exploration. Which is sometimes exactly what is needed.