Make a book safe with a secret compartment that needs a key to open it.
I made my first book safe when I was about 10. I still have it, and although it's pretty rough, it still does exactly what it's supposed to - hide things :)
I now own a lot of books so it's pretty easy to hide my little safe amongst the other ones on the shelf - (even my wife doesn't know which one it is!). If you only have a few books on your shelf though, then you could be in trouble as there's a chance that someone might come along and find it.
I waned to improve on my original one so decided to add an extra secret section which would need a key to open. The extra section is located behind the initial"safe" in the book, and is kept hidden by the home-made lock and key.
It's quite an easy project, it just takes some time and patience preparing the book.
Here's a short video
If the above doesn't work try this link