A few teachers I am working with are doing a roller coaster project. This can be a tough one to pull off especially if you are focused on having the students demonstrate how their design decisions were impacted by physical science concepts such as potential and kinetic energy.
To get your students fired up about coasters and to start learning about physics, I recommend Disney's Imagineering program. The website has a great user interface and the resources include a roller coaster build-simulator app as well as an educational DVD on roller coaster science that includes companion experiments. The roller coaster app has one level that is free. The other resources you have to pay for.