We put together an "invention box" shortly before I went in for the slice n dice. An invention box (or in our case basket) is a place where you can store the tubes out of wrapping paper rolls, empty milk cartons, egg cartons, scrap material etc etc. The basket is a creative free-for-all for the kids. They can dig through it and make awesome inventions out of items that would otherwise end up in the recycling or a landfill. I also include items like tape and glue sticks. It is a super easy thing to do, grab a basket or box and start filling it with interesting items that will get their imaginations going.
Below is a circus we made out of old boxes and birthday tags, and a board game that I had picked up but never did anything with. They seem to be as entertained by this as store bought items. More actually because they get to engineer the whole set up (with a bit of art direction of course).