Beer. Not to show, just to enjoy it.

I'm an antenna engineer. I'd bring an antenna because after hearing a lot of adult show-and-tells I figure people could probably use some sleep.

my laptop so i can show off gnome and emacs (and my trackball!)

I play Warhammer and D&D...and have extensive mini collections. I'm envisioning a diorama.

Or a few of my favorite knives and a good whetstone, to show how easy it is to do and teach a good skill!

If we could go outdoors I could probably bring my bootleg pelican case full of ham radio equipment, deploy an HF station and try making some contacts while explaining how each of the parts works

I'd bring my Split ergonomic keyboards I've built/am working on.

I'd love to get others into the hobby or crowd source help in coding QMK from those who are already in it. Online forums are a great resource but for some there's no replacing an in-person teacher.

I'll hopefully come back and edit this post when I get home to share a picture.

I own a sextant. I have other cool stuff, but that's the one that probably has the most universal appeal.

This has been a very cool thread.

I'd like to bring a tall ship sailor. Preferably from the 18th or 19th century. I bet they'd have some interesting work stories to tell.