The Mt. Diablo Summit Trail is a lovely attainable trail that works its way up the mountain. The beauty of it is that you can do it in sections because it crosses the road to the summit in multiple places, and there are multiple parking opportunities.
However, there is a section further up the trail that is very brushy. The park has spent a lot of time cutting the bushes back, but it's narrow enough to still not feel very safe during COVID. Conveniently, there is maintenance-vehicles-only road that by passes the section. It's called Green Ranch Road and it is sign-posted. Make sure you only do this section of Green Ranch Road as the rest of it heads in the wrong direction. One you hit the restroom, switch back to the Summit Trail again.