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!
Step 3: Reassembly
1) Glue the brass nozzle back into place in the white rubber gasket. Make sure it's centered as not to draw attention to scrutinizing eyes.
2) Glue the nozzle-gasket assembly back into the black plastic ring. When you do this, take care to not leave any glue on the threads of the ring, as you'll need to thread it back on to the body in a bit.
3) Dime time! Push your dime into place on the bottom of the sprinkler to cover the hole, then fill it over with plenty of hot glue.
Step 4: Final Steps
An addition to this build that might be worth looking into is adding some vertical fins to the body of the sprinkler, as I found that if the cap is put on as tightly as it should be, the entire sprinkler has a tendency to spin in its hole. this could be as simple as epoxying some scrap plastic to the sides, and probably doesn't need to be any more complicated.
I hope you've enjoyed making this; please comment and rate!