One of my favorite "century" bike rides is the Surf City AIDS ride from Santa Cruz in the fall. It's a 100 mile bike ride in a single day that benefits the Santa Cruz AIDS Project. I have ridden it three times before, but have shied away from the fund-raising aspect of the event, preferring instead to pay a flat fee and just ride. Last year I skipped it altogether, as I was recovering from a crash. This year I've committed to raising $250.
If you would like to learn more about the ride and/or consider donating, click here: Surf City AIDS Ride 2011 - Santa Cruz AIDS Project
Of course, it's not too late to register as a rider, and I would be glad for the company. (There are routes shorter than 100 miles.) It's a beautiful course and well supported by friendly volunteers.