I'm a real fan of Camelbak so storage is not an issue. I don't like large seatbags on my bike. They can interfere with pedal stroke. I do have large Bento Bag which is perfect for my camera or cell and keys.

Also, if you have an extra water bottle cage, you can use a empty bottle (with a large mouth) for storage. It's a great place to keep your extra tube, tire irons, patches and bandaids, cleaning cloth, etc. Just make sure its packed so the stuff inside doesn't rattle and drive you crazy.