I did a logo challenge 3 years ago. As many logos as you can produce in just 3 days in your spare time.
Let's face it, some of them are pretty bad. In the end it was getting pretty stale and I lost my creativity but it pushed me to work faster and really use Ai's shortcuts.
Update: I'm also adding some logos I did recently because I don't know where to put them. (the Allin mobile one and the Andrea D'Arcangelo one)