An idea I had for currency (Rather than bottlecaps) would be to possibly still use old world currency? That way people can literally become rich by scrounging (and not for taking a drink) it also makes things a bit interesting as ofc it'll eventually run out. You can also twist it that people who had no real idea which was which started using it first, so for example 50ps could be worth a lot whereas pound coins/notes are worth basically nothing.

I mean why would a note with a 20 on it be worth anything!? They're so fragile.