One Piece Deafening Horn
This is an extremely loud airhorn that's easy to print. I have found it very useful for waking up friends, and for pretending to be a bus on the street.
This is based on the design of an airhorn I saw being sold on the street near Fisherman's Wharf in San Francisco. The geometry is simple and clever.
After your print it, just add a balloon and a rubber band, and you're good to go!
Made in OnShape:
https://cad.onshape.com/documents/fc84b201b8f848d88d503020