I started out with something most people can relate to: different types of documents. You can create a generic Document class, and then a series of subclasses: Letter, Memo, SummaryReport, DetailedReport, Thesis, Resume, Timesheet, LetterOfComplaint, whatever.

In my case, I made the code handle a lot of formatting tricks, and produce output in a variety of formats.

Whatever you choose, I think the key is to make it something you might actually want to use. That's the only way you'll keep using it in the long run.

buckaduck