I use ZipTies.
I carry about 10 of them and they weigh nothing. If I’m popping into a shop I tend to walk in with the bike, they moan on occasion but I simply state I’m not leaving it outside unattended, once they’ve seen the kit on the bike often they give in.
But back to the ZipTies, if you link a few together they lock, don’t do them up tight unless you fancy cutting them off.. But if you zip them up loose they’ll hold any bike to a post or such whilst you pop into the shop, then just use the end of another ZipTie in the clasp of one of the ones you’ve used and it’ll open right up to be used again.
I put them into the bars on my bike using the bar end plug and a bit of tape to stop them rattling around.
Other than that I do have a Cafe’ lock, small and neat wire & coded lock that fits into the front bar bag, but even that isn’t robust enough to leave even for the shortest of times. I don’t tend to use this very often, it’s ok for remote places I guess but if you are staying over in a B&B or such and the landlady who said No to bringing the bike in the room I then take the wheels off & attach them and the frame and carry the whole lot into the room.
I’ve only been told not to do that once, whereby they locked the bike up in their garage which most folks do anyway.
Can’t say I’ve ever fancied roadkill.. but I have drank stream water with a steriliser tablet in..