In this video, MonsterCatAmbush in Bloomington, Indiana gives away the secrets of his latest geocache. He built a very sneaky birdhouse geocache that you can't open... that is, unless you solve the puzzle and let the birdy help you find a way in.
If you live near Bloomington you really need to check out this video. Of course he only shows you how to open the geocache - not how to solve the puzzle. For that you'll need to do some ornithological research.
This is another example of a high quality geocache. Hopefully you find some inspiration for your next geocache. I know of at least one geocacher who was inspired to build a more creative geocache since watching MonsterCatAmbush.