A film canister holds your wound-up roll of film inside to protect it from light, and you can use that same mechanism to stash a cute DIY invitation inside.
Do you still use film for your photos? If you do, chances are when you get the rolls developed, those empty canisters are landfill-bound. Next time you drop off film to be developed, see if they'll set aside the empties for you to reuse instead! If you ask super nicely, maybe they'll collect other folks' empties for you, too!
You can make this project from the folks at Photojojo even more eco-friendly by using reclaimed or recycled paper to make your invitations instead of new. Most junk mail probably won't work for this project, because that flimsy paper might tear when your guests unroll their invites, so opt for heavier-weight reclaimed paper like security envelopes or fliers printed on regular paper rather than newsprint.