The Scratch web site – http://scratch.mit.edu/ – has thousands of examples which may serve to motivate and inspire students, as well as extensive tutorial and learning materials.
Suitable ideas for this themed competition include interactive/animated presentations, games, simulations and stories.
A quick browse of the Scratch gallery of examples will no doubt produce many more possibilities.
You can download versions of Scratch for Mac and Windows. Scratch will even run off a pen drive, thus overcoming any security restrictions on school PCs or laptops (see http://www.scratchguide.com/run-scratch-from-flash-drives/ for further details).