Make a Treasure Map
Having several hiding places makes sense...unless you forget where they are! Avoid this misfortune by making yourself a map of your various treasure sites. That way you only need to remember one location—the place where you hid the map.
The Old Hollowed-Out-Book Trick
We've all seen the hollowed-out book, but there's not much room in one of those. Instead, use several books with a plywood box attached to the back. If you have a band saw for cutting out the pages, great. If not, you can use a jigsaw. (After all, books are just a form of wood.)
If the sides of the books will be visible, fold back the covers of the books on the left and right sides of the assembly before cutting. Build a plywood box to fit the opening and glue the book parts to the box with construction adhesive. The disadvantage? You can see inside the box on low shelves, so you need to display it so the opening is above eye level.