"Readymechs are free, flatpack toys for you to print and build. They are designed to fit on an 8.5"x11" page and printed with any printer. You’ll need double-sided tape, thick matte paper, and 10-15 minutes for build time."
They have some bad-ass designs here. I haven't tried it out yet, but this would be a cool project on a rainy day, or something fun to share with someone when you want to be silly.