If I were you I would keep it very small and inexpensive. People donate because they really want to contribute, not get something of value back :-)
I would be surprised and happy to receive maybe an attractive card saying something about the ride, or just a single piece of quality chocolate with a personalized wrapper.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett