My son made a slot machine.
This is a great hands-on thinking project especially for those future engineers who love to tinker. There are not right or wrong answers, just a million different ways to do something similar. In fact, the challenge can be modified to meet the project goals too.
Using cardboard, tape, glue, string, straws, and whatever else you want to include here (recycle bins are wonderful), ....................
Create a machine that will randomly return coins.
Create a machine that will sort coins based on size.
Create a machine that will return one item of the customer's choosing (vending machine).
Here is one my son created.