Hole in the Door
Drill a hole in the top of any interior door. Size it to fit a cylinder such as an old film container or a cigar tube. Roll up some bills and keep them there.
Editor's Note: If you want to do this trick on a hollow-core door, you have to stick close to the outside edges. Look at the door from the top and you'll see how wide the solid internal frame is.
Hide a Key In Plain Sight
Say you want to hide a key—other than under the rug or over the door. How about mounting a phony plastic LB fitting? Screw it to the wall and run a bit of 1/2-in. conduit to the ground so it looks official. Cut the head off the bottom screw and glue it in place. That's it. Swing the cover aside and there's the key.
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.