It would be fair to call the extensive rock formations at Mt. Diablo State park a hidden treasure; at least, in the context of everyone's focus on taking photos at the peak. But, my advice is not to no take Rock City for granite for it has great sedimental value (ok no more puns).
Sure, Mt. Diablo has a peak of nearly 3900ft to ascend, 200 bird species and in excess of 400 flower species, extensive recreational use. However, I have been most impressed with hiking and exploring around Rock City's numerous rock formations, Indian grinding rocks, and trails. You'll find all the geological wonder a short drive from the gate near the top. Once you start exploring on foot you'll notice nook and crannies formed over millions of years. Wind caves are almost tunnel like crevices in the rocks, once below the water, that formed by the wind after the water was gone. The ancient vibe can be felt for sure; you might find fossils and there have been discoveries from the indigenous people (i.e., Ohlone, Miwok, Volvon, and Nisenan people) who once lived here. For more info see: https://savemountdiablo.org/blog/a-brief-history-of-native-peoples-around-mount-diablo-and-treating-their-cultural-artifacts-with-respect/ The big draw here are Elephant rock and Sentinel rock; you can see both on a 1.2 mile that starts at Elephant rock. On clear days you can see incredibly far.
I suggest being really really careful when scrambling over the rocks and exploring wind caves; don't be stupid for that social media shot. Definitely visit in spring when the flowers are out. Entrance fee is 10 dollars and dogs are not allowed. There are many places for a picnic but bathrooms were sparse. Kids will love it here but watch them carefully since there are many dangerous falls if you're not careful. read more