If you're contemplating using Tk, I'm going to assume (improperly, perhaps) that you've introduced them to the concept of libraries, maybe writing their own object classes.

What I would find fun is to write text adventure game, where groups of students handle different parts of the game (the parser, the database, maybe a templeting system for the printed text). I would handle this by writing the main code myself, and defining all necessarry module API's as the instructor. Then, give each group the task of building a library or a class that implements the functionality and the interface your main code deams neccessarry. Perhaps you'd have to provide them with the data - the dictionary for the parser, etc.

I think that this would allow them to bring together everything they know, and see how modular programming allows large groups to work on a single powerful applicationn without getting in each others way. Also, I think that it would make for an interesting microcosmic view of the open source community, working together on large programs, patch by patch. They could review each others code also, giving them debugging practice.

I'd enjoy doing that project in class!

Cheers,
Erik