I use a Patagonia Atom sling. Big enough to hold what I need, easy to access the interior without taking it off (just rotate it around and access it while it's still on), and I barely notice it's on while I'm wearing it. For many rides, you don't even have to take it off, just swing it around so the pack part is in front. For the thrill rides, it's small enough to either fit in the pocket of the car or just hold between my legs.