I have a Bontrager trunk bag with fold out panniers. Since I'm almost always biking with my kids, and I like extra ice water even when I'm not, I use it all the time. I usually have one or two 40 oz vacuum insulated water bottles in the main pocket. I keep a spare tube, pump and patch kit in there for any longer rides, and snacks, sunscreen, bug spray type items for the kids most of the time. Extra clothes, and swim suit go in if I'm biking to the beach. I've also packed and carried lunch with freezer gel packs to keep it cold. Also comes in very handy to carry things back from the farmer's market or smaller grocery runs. I get odd looks when I don't bother to take the bag off when I hit the trails, but I'm not trying to impress anyone. I already have a 40 lb bike so if I carry some extra weight in supplies, all I'm getting is more exercise which is the point anyway.