Definitely check out the Yuba River. With beaches and boulders, this river offers beautiful swimming holes, wildflowers along the banks, and great hikes.
As a small-town escape for culturally creative folks, Nevada County's also got some great festivals: Music in the Mountains, the Bluegrass Festival, Cornish Christmas, Victorian Christmas, the Storytelling Festival, the Literature Alive Wordslinger's Festival, etc. plus some interesting Theater.
Quick Tips:
Summer is hot (95 degrees is not unusual), Winter it can snow. Beautiful seasons, but pack accordingly.
Best Way To Get Around:
Possible to get there via Amtrak (bus from the Sacramento train station) and Greyhound, but a car is helpful around town as well. Like many rural communities, it's small but spread out.