There are so many people in the world designing stuff that, with limited resources, it's quite difficult to find a concept that no one has ever built before. Asking other people for ideas won't give you designs that have never been done before either...perhaps it would be best to pick a neat project rather than one that has never been done before? For a school project with very limited resources and time...to find something that has never been done before seems like a pretty ludicrous criterion.
That said, I think undulating snake robots are very cool (they have been done before, but probably not often). Lots of repeating segments can make it easy, but many motors can make it expensive.
There's also the good old inverted pendulum machine- recently a two-wheeled segway-type robot has been a popular version of it.