I have a fair amount of crap, but not a lot of it is of much interest to most people.

Unless someone out there wants me to show up with a laundry basket full of Fumos and subject them to an unsolicited three hour lecture on Touhou lore...

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My fully water cooled itx PC running arch, then I can force my hubby on them 😍

Probably a synthesizer. MEGAfm would be the coolest, but I imagine my desktop Hydrasynth would be more impressive.

I'd bring my Moog Theramini. Nothing like playing the air waves! It's magic science!

Not sure what a show and tell is, I guess my laptop with tons of stickers, and boot up something that looks hackery. Besides that, my guitar perhaps?

I'd bring some beer that I make in my garage.

Probably either a Ken Payne bronze called The Trail of Change or my Ibanez s521 guitar with the ocean fade paint job.

Inherited it from a family member who loved southwestern and native American culture and history they shared with me so it's some sentiment along with an interesting statue.

Most of what I own is minimal and function over form but I just think it looks awesome and is fun

Climbing gear or 3d printers.

My knives, because it's sort of weird even for me that I carry these knives around for work, and a weird thing becoming humdrum is the sort of thing I'd like to hear about at the show & tell.

My autograph collection.

Professional level landscaping accessories.

Folks always seem fascinated by my keyring.

My papercrafts.

Depends, is it a casual thing or more serious? If it's casual I'd probably just bring a Trading Card Game deck (MTG, YuGiOh, Pokémon). If it's more serious, I'd bring in some of my junk like a broken microwave magnetron, to simultaneously bring attention to the problem of short product cycles, explain a bit about how what most people see as a magic appliance functions, and show that even complex things can be broken down into simple parts.

You know, I think an adult show and tell wouldn't be too different to a kids one, we don't change that much in what we find cool from 7 to 70.

I'd bring some pets, I have geckos that are an all female species and reproduce only through parthenogenesis, most people find them cool.