Ugh, I hate these things with a passion because even if it’s not required, if a bunch of people do (especially management), they’ll often feel the ones who don’t participate are “no fun” or “aren’t a team player”.
The only one I’d not seethe at is the Mad Men suggestion.
animal print day?
Bang goes my suggestion of black tie and ball gowns.
How about a Hawaiian shirt day? Prizes for the loudest / most garish etc.
Or a loud tie / headscarf day?
One of my workplaces used to do this sort of thing every now and then. Once, they had a medieval theme and a friend went in in a suit of chainmail, battle axe, and all.
But I suggest you have several ideas and make each a one-off, every other month at most, and have a lunch which follows that theme (e.g. Hawaiian).