Make a Calculator that accepts different inputs like polish notation, reverse polish notation, normal notation. You could support parentheses and the basic operations like plus, minus, multiply, divide, maybe even mod and pow. This will cause you to use a couple basic data structures like the stack and bonus points for parsing.
You could also go with AST (Abstract Syntax Trees) where you’ll use… Well… Trees. Trees are quite interesting, and there is a variety of them. Look them up, see what they are used to do and maybe you’ll find some ideas to… Borrow.