I thought an ordinary can of Baked Beans might make a good hiding place, for example, for cash, if you keep some in the house.
This Instructable uses a special bit of kit, the Kuhn Rikon Lid Lifter can opener (as recommended in the excellent Cool Tools).
As you will see in these photos, the 'lid lifter' cuts into the side of the can's lid, which means you can lift the lid off (and also replace it again if you like). It has the added advantage of leaving the top of the can smooth, not sharp.
When you've removed the lid from the can, clean it out and dry it. Then you can fill it with whatever you like, and replace the lid. In this case the can is the 'ring pull' type and I have removed the bottom. When the can is back on the shelf you can't really see which one is the secret one. With something light inside the can stays shut very easily.
There is one shortcoming of this, of course, it's the weight. It doesn't feel anything like the weight of an actual can of beans. I expect with a bit of plaster poured in there, or similar, you could make a closer approximation to the weight.