If your looking for a sneaky geocache hiding spot that will be a challenge to find, the sneaky fake birds nest will do the trick. These are really fun and very easy to setup. Most craft stores sell fake birds nests in a variety of sizes. All you need to do is attach one of these nests to a tree in a natural looking way and place a small geocaching container inside and you will have a tricky little geocache that your hunters will have a great time finding.
For some extra sneakiness, you can even purchase a couple of small fake eggs, I have seen some plastic one that come apart, just place a small log scroll in one of the eggs and your sure to have a few geocachers scratching their heads.