Cut out a stud space opening to fit a return air grille. Cut off the grille screws and glue just the heads in place. Run four drywall screws into the corners of the opening so they fit just inside the rim of the grille. Then glue rare earth magnets to the back of the grille so they line up with the screw heads.
False Stair Tread
It takes some effort, but if you can, free a tread from your stairs. Then attach a piano hinge to the back. It'll be almost invisible and you'll have a good place to stash valuables.
Roll up some cash, stick it in a medicine bottle or any other watertight container, and bury it in a potted plant. For quicker access and to keep dirt from getting under your fingernails, place a stone or pine cone over it. Not many burglars are going to be excavating around your houseplants.