Testy yet trying: Car Ride Activities Set 5 - A good list of busy bag ideas, 7 or so pages of activites that can be put into bags for road trips or anytime kids are needing to be confined (weddings, appointments, etc.)
Just stick a few long bolts and matching nuts and washers in a bag. They screw them on and take them back off. They try to see how many nuts will fit on one bolt. They can make a pattern (one washer, one nut, one washer, one nut).