Check out the bike path that runs along Corte Madera creek. It starts at Creek Park across from Marin General and runs through College of Marin and on into Ross.

Also try Bicentennial Bike Path which goes through Bothin Marsh along Richardson Bay near Mill Valley.

If you've got mountain bikes, a good flat place to ride with the kids is at the Las Gallinas Valley Sanitary District wildlife ponds. Lots of level gravel trails: http://www.marinmommies.com/hiking-kids-las-gallinas-valley-sanitary-dis...