Waterproof, inexpensive, inconspicuous, and quick to build - let's go!
Step 1: Stuff You'll Need
-A dime. Yes. A dime.
you'll also need a hot glue gun, but that's it!
*This might seem obvious, but it's generally good practice to find a sprinkler that your keys will fit inside of. If possible, open up the sprinkler you plan to purchase/use to see if your keys will fit. If you have a car with a large key fob, you might have a harder time finding the appropriate sprinkler.
Step 2: Disassembly
1) Unscrew the black plastic ring
2) Pull out the entire sprinkling (?) mechanism
3) Unscrew the brass nozzle
4) Slide off the white rubber gasket.
The parts you'll need to keep are the black plastic ring, brass nozzle, rubber gasket, and body of the sprinkler. If you come up with something useful to do with the spring, lemme know in the comments!