[bmelton]

A static, Jekyll-like blog generator would get you a good grasp of language fundamentals I'd think, even though at least one already exists for Flask.

A message-board/forum/discussion type piece of software will get you a solid grasp of SQLAlchemy and company.

Something I'd LIKE to see in Flask (that I don't believe exists yet) is some sort of analytics package that dumps data into a Mongo/Redis/NoSQL store and presents things in a pretty package on the backside.