There is a flat (but pretty boring) ride you can do from Lafayette to Danville/Blackhawk/San Ramon and back. Several CoCo riders here know the specific route if you want to venture that far out on BART.

From SF, you can do a nice ride of about 80 miles to Point Reyes Station via Nicasio. It's not flat, but the total climbing is probably only about 3000-4000 feet in 80 miles. I have the route if you want it.