I saw a sprinkler head hide-a-key on Amazon yesterday so I thought it would be fun to make my own. I have a bunch of leftover parts from when I put in my sprinklers, so it was pretty easy to make.
I used a rotary sprinkler nozzle and plugged the bottom with a plug from a different sprinkler head I had. I opened the main sprinkler up, removed the insides and punctured the filter to hang my keys from it. After that I buried it in the yard for a very incognito backup key hiding place.