There are a number of different options for this assignment. Pick the one that fits in with what you are planning to do for your project. If none fits, do the Scheme interpreter.
Map Analyzer (MUD/Text Game) - By this point you certainly have a map where you can have rooms with connections between them. In a formal sense, your map is a graph. For this assignment you should add some commands that will help analyze your map. You should be able to do things like check if two rooms are connected and the "distance" between the rooms. The code to do these commands will be recursive. You might also want to do things like find all paths shorter than a certain length or short cycles where people can run in circles. You should make it so that if I run the server and connect then type help I will get a list of all commands, including these new ones.
Web Spider - This is pretty much what it sounds. You can play with this some to make it work better. For this version use recursion and set a maximum depth that you go from the start page. It should report links and titles of all pages that it can get to from the start.
Note that most HTML is not properly formatted XML so you will have to do a bit of work searching for instances of href to find links. The XML parser won't do all of this for you unfortunately.
Full Formula Evaluation (Mathematica) - Take the recursive forumla parser and make it stronger with things like trig functions and the like. Put it in your GUI so that the user can interactively evaluate formulas. They should also be able to set variables for those formulas. If you are feeling adventurous, put in some basic function plotting.
Scheme Interpreter - Do problem #3 from this problem set.