The other option for supported rides is a handlebar bag. For stability reasons, you don't want to put a lot of weight up there; but on a supported ride, mainly you'll want a map pocket, a SMALL camera, arm and knee warmers, a change of eye protection (clear or amber vs. dark lenses), maybe a change of gloves (fingerless vs. FF), and a little bit of whatever food or supplements you need in addition to what the ride provides. Reading glasses, if you're my age. Keep it under 3#. A map pocket is really nice to have, especially if you don't have GPS.