If you mean a project for a senior in secondary education who's had one or two courses in C++, then I had some fun recently working through the concept of line sweeping to perform tasks such as finding the two closest points in a group of points or the area of union of a group of rectangles.
If you're talking about a senior in college who has supposedly developed a fair amount of knowledge in the topic, etc, then you'll probably want something a little more involved. Say maybe using an A* algorhithm to find a solution that is gauranteed to be take the least number of moves possible to solve an arbitrary Rubiks Cube or using state systems to clean up radio signals. You know, something with a little more pizzass.
You'll probably get some better ideas if you are a little more clear on what you want.