I used to carry stuff around (tiny flashlight, Leatherman squirt, key-shaped multitool), but I ended up never needing them and them just bulking up my pockets. Nowadays, all I carry on my keychain that’s not a normal key is a Yubikey (hardware security key). I don’t need the Yubikey often, but this ensures I’m not locked out when I’m for example visiting my parents and forgot the main key. Or if I’m on my phone.

edit: Added normal because it was annoying me that I said the only thing not a key is a key …