Meet in the parking lot near Chain Reaction Bicycles in Los Altos. Ride a variant of what is known by cyclists in the area as the "Portola Valley Loop". Stop for lunch/break at Robert's Market in Woodside at mile 18. Total distance 32 miles. The route is subject to input and change, but the route I've planned can be found here: http://ridewithgps.com/routes/4712568
Thanks to Mike B. for the awesome suggestion!
Note that although there is some climbing, it is not as much as the total shown in the link and climbs are not long.