Have a bunch of these lightweight plastic retractable key chains, belt clip, soft plastic snap in. reel cord length 35 in.
They seem they would have a lot of interesting MYOG MUG potential.
My ideas are kinda lame, and need your ideas.
1) retractable lanyard for map, compass or GPS, so as not to lose when they fall out of pocket or strong wind.
2) some innovative booby-trap to scare the squirrels from messing with your food/gear
3) some trigger / release / recoil poorman alarm (noise maker) system with a few pennies in an altoid tin or an empty soda can. If you are the paranoid type while solo camping and sleeping.
4) some crazy david/goliath sling
5) snare for rodents
6) camelbak hose auto-retractor instead of the dangling hose dribbling on your shirt.
7) an ineffective food bag piñata
8) car key holder... really?
9) hole punch your credit card, not where the magnetic strip is, but near the bottom name embossing, thread the CC into the round keyring. retractable CC, if you are worried a squirrel is going to pick your pocket on a thru hike.
10) Your suggestion goes here.