This is a great loop beginning near Napa Valley College heading up and up and up and through a ton of fun stuff in Skyline.
Horses are out sometimes so always be mindful around blind corners.
Once you finish the River to Ridge Trail
, the climb up Buckeye is a lung-buster. After that you'll have some more climbing before you descend towards Lake Marie. The Teuter Loop is a must-do, especially since the way back down is fast and fun. If you cross the creek again on your way back from the Teuter Loop you've gone too far.
Heading up Chaparral Trail is a tough challenge, but the impossibly steep rock can be cleaned.