A sturdy keychain that has a hollow shaft just large enough to insert a rolled up bill (Whatever denomination you choose). The hole in the bottom is a bit smaller than the main shaft so when you put the bill inside, it will expand slightly, lock into place, and never fall out. Bill will be there in an emergency when you really need the cash (Or give it to your kids and don't tell them the secret until you get that call that they're stuck somewhere). The groove near the bottom is just wide enough to insert one of your keys to break off the bottom, revealing the bill in an easy to remove state. The fact that you have to break it, should dissuade you from using it before a real emergency. The bottom is pretty sturdy so it shouldn't break off prematurely. Feel free to modify to give it some more flair. A rocket with engines? Who knows? Be sure to print at 100% fill for maximum heft and durability. I had originally planned to fully encapsulate the bill and use the pause function to insert the bill while printing. Opted against it for 2 reasons:

1. I had my pause and remove print head clear of the print set incorrectly and my printer made a grinding sound that I never want to hear again
2. You can give it to friends and they can put their own darned money inside. Maybe they're high rollers and go for a Benjamin Franklin!

Tips on rolling the bill
Start out with a crisp, unfolded bill and roll it carefully without adding creases or wrinkles. Once it is as tight a roll as you can get by hand, lay it flat on a hard surface and apply pressure with your hands while rolling it in the same direction. Stop occasionally while it's still a bit loose to make sure that it's not spilling out the end like a Chinese Yo-Yo. Once you get it nice and tight, it should fit in the 5mm diameter hole in the end and expand enough to keep it from ever falling out.

I designed the basic shape of the keychain in Fusion, I imported the fusion design into Sketchup where I added the shaft and the breakaway groove and finalized the length dimensions. Feel free to edit any of the files to suit your needs. Be careful not to make the support for the breakaway cap too thin or it will likely break off prematurely. The groove needs to be about 2mm to fit most standard key widths for easy removal of the breakaway cap.