A penny bridge!! It looked pretty awesome and I just happened to have a whole bucket of pennies sitting in my room so I tried it out.
It was extremely mentally stimulating and a fun thing to create... it was like a really difficult game of Jenga. Mine only fell over once in the first few minutes, but I finally got it looking like this!
This would be a fun activity to do with older kids, maybe to see how big you can get it or who can make their bridge first. It takes a lot of concentration and a steady hand, so it wouldn't be great for little kids.
Try it out! Let me know how you did!