A group of artists and I started a blog (Purgatory Sketch Club) where we do these "challenges" every month. The challenges range from Cowboys and Indians to 80's cartoon characters. It's a fun way to get each other motivated and create new art.
These are some of the images I've created for the blog challenges. The top image is a group of guys in line at a comic convention. The middle image is the Transformer known as Starscreem. The bottom image is an Orc, ready to lead his army to battle.
The challenges are fun because they're usually something you wouldn't draw. It pushes you to explore more with your art. It also helps you create work for a portfolio if ever needed.