I have kind of seen this idea before, but this video tutorial below is great at showing just how simple this is to do, and how effective it is for keeping small valuables hidden and safe. You could have several of these throughout your house.
How To Make A Super Secret Safe - For Less Than $3
Another tip – if you position your safe close to the ground, on the ground floor, you can do away with the blue box and fit loads more stuff in the safe – just reach inside the wall and placing the items on the floor between the partitions in the wall. Just make sure you can reach them to get them out! Anyway, plenty of food for thought…