Note: I have provided a web link inside the project outline showing you how to build a mousetrap car, but as this is an engineering desing challenge I usually let my kids build whatever they can dream up using provided equipment.
This activity will allow you to set up an economy based challenge for your students, but does not give you step-by-step instructions for building the cars. Your kids will need to use their imaginations (and/or the internet) in order to do that.
I came up with this modified mousetrap car lab because I wanted my students to have an idea of what it's like to be an engineer with access to limited equipment and operating on a limited budget when asked to produce a product.
Students are awarded 1,250 credits at the start of the project, and they have to design a mousetrap car by purchasing items from my store (each item carries a credit cost). The store only has limited quantities of materials, so once an item is sold out the groups are forced to come up with alternative designs. As an added challenge, any unused credits can be traded in at the end of the project and awarded as extra credit points (10 unused credits become 0.5 extra credit points).