If you're a fan of dedicated bike paths, and in the area, don't miss the opportunity to ride Lafayette-Moraga. Almost the entirety of the path is along a shaded corridor, gently curving back and forth, beautiful homes visible in wooded settings, quirky sections (e.g. a brief segment with many dozens of miniature bird houses hanging from trees), St. Mary's of Moraga's beautiful campus visible at one point.
I'd recommend starting from the Lafayette side. This trail is not flat: the out and back registered 600 vertical feet on Ride with GPS. Starting on the Lafayette side will result in about 5 miles of mildly climbing trail, a descent of about 2 miles at the far end, and then a somewhat steeper (but not steep) climb back to the summit. The last 4.75 miles of a round trip to Lafayette involves a swooping and delightful descent back to where you started. While the climbing is not negligible, it is well within the capabilities of even moderately fit riders.
My partner and I rode this trail on our tandem trike on a Wednesday, finding light to moderate foot and bike traffic. If you're family oriented, use caution with the younger riders, as on the downhill slopes some riders will be coming down in excess of the 15 mph speed limit.
Overall, a gem of a ride, scenic, interesting, and just enough of a challenge to feel that your heart and lungs benefited from a great day outside.