If your folks are in Nevada City, I'd suggest Auburn.

While only, what, 30 miles east of Davis, it 1000' higher and defiantly in the foothills with some killer climbs. It snows a couple times a year, but that won't stop you. From Auburn you can either go west and very quickly hit the flat stuff or any other way and get some climbing in.

You probably already know there is some great kayaking and hiking along the American River there too.