Great job thinking outside (or in) the box!
If you can get your hands on a very large box, you can reinforce it with extra tape, cut out a door and windows with a box cutter and them let him decorate it with paints, markers and stickers.
You can make mobiles with cut out shapes and hang them on wire coat hangers - teaches balance, think early physics.
Very little kids can make, with help, terrariums out of glass Mason jars or old glass canisters.