Two things:
One, I have never understood questions like "please give me an idea for a project about <topic>". Wut? I always have far too many project ideas, not a shortage. How did you cultivate your shortage of project ideas?
Two, you could make a RPN calculator implemented as an FSM. That way you could implement a simple ALU on the side as well, showing of your $k!llz. And by picking the type of calculations you support in the RPN you can put a limit on the amount of work you have to do for the ALU. So if this is a 1-2 week project you will want something a simple ALU. And if this is moooooonths then something a bit more elaborate might be in order. But I take it you will have a design review before you start coding?