Hide a Safe in the Wall or Floor
Install a wall safe or cylinder floor safe (a SentrySafe Floor Safe is available at amazon.com) by bolting it to the floor (most safes have holes inside for just that purpose). Hide it in the corner of a closet or other inconspicuous area. Or mount the wall safe inside a wall and cover it with a picture. Or chip out a hole in your concrete slab and stick in the floor safe, then pour new concrete around it.
False Top (or Bottom)
When you build a piece of furniture, build in a stash spot. For example, when you assemble a dresser, put a piece of 1/4-in. plywood just above the top drawers and install a piano hinge on the top. Now you have a spot to hide precious items.