30 mile loop with climbing, start/end location = Stanford Mall. Non-cycling g/f may be able to amuse herself there for 2-3 hours.
http://g.co/maps/jh3d3 or a little longer
http://g.co/maps/vsam9
Racer Ex's Tunitas Creek is better, but it falls into the multi-hour epic category IMO. There's endless multi-hour epic climbing rides on the peninsula.