Stump Them. Maybe Even Literally.
Use your vertical space. Those treasure hunters are going to assume that the hidden gems are on the ground at the coordinates that their GPS directs them too. Stick it up in a tree or make use of a cliff face by placing it in a crevice. Of course some of these suggestions could get you hurt so don't do them if you're unwilling to accept that challenges in life can be dangerous and you want someone to blame when they happen. But if the vessel which is you wasn't built for any safe harbor, push the edge and head off for those proverbial unknown shores through stormy seas. And if you're really motivated and slightly morbid to boot, bury that thing six feet under. Perhaps all that effort uncovering the geocache might teach a person how valuable every moment of life is since we are all inevitably headed there someday.
But let's get back to the point of all this, which is having fun in the great outdoors using that nifty and ever-improving GPS unit for tracking. Use the tools of cover and concealment; meaning cover up that stash with anything available, foliage, grass piles, branches, and pine cone heaps. You can even buy a spray on adhesive that you could coat the ammo can (treasures are often located in an ammo can) and then attach dirt, leaves, or anything else to really blend it in with the environment. Stuff it in a decaying stump. If the stash is to be placed somewhere in a neighborhood, take advantage of the ordinary stuff you might find in a yard which may seem like an unlikely candidate for a geocache housing apparatus. Perhaps a bird feeder, an operating sprinkler, or inside one of those fake rocks you can purchase. Or under a really heavy rock.
Now I'm sure there are also jaded pranksters among you who would be tempted to indoctrinate the dated waste of an animal like a cow or a dog but I'm not condoning that at all, unless of course, the shenanigans have already escalated to that level. What about bodies of water? Why not? Just place the treasure in a water-tight box and place it in a stream where it won't be carried away or if you're really cruel make them bust out the scuba gear to find it in deeper water. Or make it so they have to find it at night; that brings a whole new level of excitement to the game. So just use your imagination and you'll find that hiding the geocache can be just as rewarding for body and mind for you as the finding will be for them.