Toys for Animating Mathematics
The aim is to take some mathematics that would be within the grasp of a mathematically-inclined sixth-former and turn it into some attention-grabbing web pages. Ideally this reveals a connection with computing science. I imagine that this project necessarily involves some sort of animation, and I have visions of Open University television maths lectures.
The programming need not be the most important part of this project, though, because some of the work is in choosing a topic and designing problems and puzzles and the like around it. There's a lot of this sort of thing about, though, so it would be necessary to be a bit original.
Think of GeomLab (and then perhaps think of something a little less ambitious). It might involve logic and proof, it might be about sequences and series, it might be about graphs, it might be about the mathematics of cryptography... you might have something else in mind.