Ace, I've been to Costanoa - there are tons of places to ride. Up and back to Half Moon Bay on highway 1 (you'll be right off the highway) would be a simple one. Do you have a Krebs map? That would show you all the rides in the area.

Here's a link to some routes in the area, starting at San Gregario. (North of Costanoa, I believe) You're pretty much in hill country, but there are probably lots of routes you could take to avoid too much pain.

http://bikemaster.home.att.net/routes/sgr_list.htm