Minimalist wallets are a great way to cut down on all the junk in your pockets, but if you want to go all in, Make shows you how to make a wallet and key holder combo out of a piece of leather.
You'll need a single, small piece of leather for this, but repurposed leather from an old purse, jacket, or other op shop find will do. Then, you'll use some waxed lacing, hardware for the keys, cheap snap fasteners and you're all set.
Obviously this will only work if you only have a few keys on your key ring, but if that's the case for you, it's a pretty handy way to carry all that stuff in one little unit. You'll find the full guide and the pattern over on Make.