I've heard good things about the Bike Around the Buttes http://www.bikearoundthebuttes.com/. The ride goes around the Sutter Buttes near Yuba City. Flat mostly, kinda low key ride is how it was described to me.

Feel free to correct me...

My personal favorite that time of the year is the Chico Wildflower http://www.chicovelo.org/wildflower.html. However, for the full century route you do three good climbs. Might not be what you're looking for, but it's a really fun ride.