This challenge is simple but can end up with surprising results: Write a song in a genre you've never written in before. Bonus points if it's a genre you've never even listened to before.
I found it really enlightening to break down elements and patterns of other genres so i can better understand my own default genre approach.
Find something that looks interesting there, then look up the wikipedia entry to find out the defining characteristics of that genre.
Then write a song with those characteristics and post it with the tag #newgenre.