Round Valley Regional Preserve is not as popular like many of the East Bay trails, probably because it takes quite a drive to get to it, but this trail is one of the best I have experienced in the Bay Area.
This is the AllTrails link for the trail. This is our Strava recording:
There is ample parking at the trailhead and a gentle climb towards the summit.
The view from the summit is absolutely stunning. You have to get off trail towards the edge of the valley to take in the views of Los Vaqueros Reservoir and the rolling hills.
Round Valley Regional Preserve is adjacent to Morgan Territory and there are trails that lead you to Mt. Diablo. I am certain there is a great 3-4 day backpacking trail here to explore this area.
If you are looking for a less crowded trail in the East Bay, this is worth the drive.