There are a number of different options for this IcP. Pick the one that fits in with what you are planning to do for your project. If none fits, do the Scheme Interpreter. All of these are basically having you refactor something you had done before using a parser. You can also try to use this to add additional functionality if you want. When you do this, don't jump straight to code. Come up with a CF grammar first, then you can put that into the code.
Command Parser (MUD and others) - If you have something in your program, that needs to parse out user written commands, try using a parser to do it.
Formula Parser (Mathematica) - Parse out your formuals using a combinatorial parser instead of doing your own recursion. Make it so that the products are a tree for fast evaluation.
Scheme Interpreter 2 - Assignment 2 had an option for writing a Scheme interpreter using recursion. You can also parse the Scheme using a combinatorial parser. That is the goal of this option.
Any Other Parsing - If you can think of something else in your project that would benefit from a parser, try it out.