Aside from my phone and wallet, I only carry my keychain. Each goes in a separate pocket.
I do keep two small tools on my keychain:
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A small, bladeless multitool. Can take on flights. Looks like this particular one is no longer being manufactured, but there are a number of vendors that make a variety of tiny keychain multitools. Just a bit over 1 3/4" long.
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A tiny folding knife, which IIRC is a Spyderco Bug that I got on sale. Slightly smaller than the multitool. I remove it if flying.
Doesn't really require much storage space.