A few friends recently put together a weekly, collaborative type challenge called Type Club 2014. They passed the link on to a handful of creatives they know, and the results have been really beautiful to see.
The premise: Each week a new quote is posted. You can interpret this quote any way you like, using any medium you like. There is a time "constraint" of one week. This "constraint" creates a deadline and a push for you to share the "final" piece with everyone else with out belaboring your work through social media.
It's a great exercise, and fun to see the interpretations and beautiful design from everyone.