Clear empty 35 mm film canisters with the lid that fits on the inside of the canister. (These are getting hard to find. Used to be you could go to places that developed film and they would just give them to you. Two websites that carry these are: http://www.sciencebobstore.com/products/Bulk-Film-Canisters-for-Rockets.htmland and http://www.filmcanistersforsale.com/ or just Google it.
Alka Seltzer tablets (save a small fortune and buy the genetic brands +/-$4 for 36 tablets) Water
Stickers and or
Color permanent markers