One of the few things I was able to locate in our moving box mess in the garage was the foam board gravestone decorations. I have a handful of them and I set them up around the yard.
I put a prize behind a couple of them and piled up some leaves on top of each prize, then piled leaves up behind the other stones, just to slow the hunt down a little.
If you want to get crafty with this - there are lots of links on pinterest on making some yourself and it is definitely not hard! Here's a few of my favorites if you want to give building the a go!
THIS ONE from Soap Deli News is made from cereal boxes. You could easily edit the original design to leave a hole to hide the treat in, and these would also be ideal if you needed to have the tombstones inside due to weather or lack of a suitable yard.
THIS ONE on Stolloween uses packing materials and boxes, and would work indoors or outdoors, depending on the size you make them.
THIS ONE from HauntedYards is made from foam boards and doesn't take a lot of taping like the previous two. Cut and paint, essentially.
Just like some of the other scavenger hunt items, this could be turned into a fun project for the kids to help make. Bonus that these wall could be decorations for years to come, if weather-proofed a little.
Here's the lot of them, set up in the yard.