I saw a good movie last night. It's a documentary called "Freestyle: The Art of Rhyme". "Freestyle" is a type of rap where you spontaneously make up a rap. The lines can't just rhyme, they have to flow logically and make sense. And you can't pre-write any of it; it has to be spontaneous. Try it sometime, it's hard! I think it could be a good songwriting exercise even for songwriters who don't write in the hip-hop style.
Rent the movie. I got it on Netflix. It's amazing the stuff these guys come up with on the spot.