On keyring, everywhere!

house, car keys (natch!)
AAA flashlight
PS4multitool
tiny prybar
tiny lighter
cash (in capsule)

out in nature...
signal mirror
Ritter RSK Mk 5
at least one more multitool
matches, lighter, tinder (plus alcohol based sanitizer)
space blanket
AA headlamp w/ spare AA & AAA batteries in capsules
700 cc cup nested on canteen (at least one)
light jacket w/hood (Patagonia Houdini)
SOL Bivouac d'Urgence
whistle
base plate compass -usually there is a topo map as well
duct tape
FAK
food of some sort
hiking staff, various types

overnight add sleeping bag, tarp or tent, stove and fuel, pad and probably more clothing, depending on situation; all kinds of other gear depending upon circumstances