How about something fun, like filesystem statistics? Keep track of the most used files to make sure you spread the disk and your mental load equally. Quite possibly useless, but could be fun to do. The hooks into the FS might be the hardest part about this, though.
Write a generic ETL app. Quite useful. Might be many out there, though. Probably few good free ones..
Or something that converts a (well known) log format into database entries for the purpose of easier statistics than what grep can provide?.
For instance, take a webserver log, dump it into the database and generate something like a visitation path..
The database isn't technically needed for this, of course, but with a large dataset, you can't keep it all in memory, so it would be useful..