Too simple then. Okay, how about Calculus: Symbolic differentiation.
For example you give the code a
string like "X - sin(X) + exp(X^2)" and you have to parse that String and output its derivative with respect to X. So the output would be "1 - cos(X) + 2 * X * exp(X^2)" (if I did that correctly in my head just now). I recall doing that as a CS project myself a number of years ago. You can adjust the number of differentiable functions you support depending on how much work is required.