We all need to hide stuff every once in a while. YouTube user supergokue1 shows us how to create a small compartment in the wall where you can stash some valuables.
To begin with, he pulled off a small piece of trim between two doors. He then cut out a piece of plasterboard to make a small cubby hole. By simply placing the trim back in place, your valuables are hidden out of site. As a couple of commenters point out, you can use Velcro or magnets to ensure the trim doesn't idly fall off. If you want to make extra sure that the compartment isn't discovered, you can re-seal the trim with the original nails. This will make it harder to remove again, but it also helps ensure no one stumbles on to your stash.
As Three Thrifty Guys points out, hiding spots like these for valuables were much more common during the Great Depression when trust in banks was eroding. You can check out their post on the subject at the via link below for more tips on where to hide your valuables around the house.