Wooden planks, all the same size, hundreds of them in one box. The first instinct is to start stacking them up. On closer inspection, you will realize that you can do more than just stack them up. Turn them sideways to make higher towers and ramps. Create parallel tracks to zoom a little plastic ball down, put walls around your tracks and how about a couple of planks across the top of the walls to create a roof?
We easily passed an hour happily experimenting with many different structures and ramps.