One of my favorite cartooning exercises is creating faces out of different shapes such as circles, squares, diamonds, triangles, etc. I find this particularly helpful sometimes because I often fall into the trap of making all my characters, particularly those that are human, look similar. A bit of "same face" syndrome, if you will. I did a male version of the exercise back in March and finally got around to completing the female one today. I actually found drawing guys using this challenge easier, which is surprising since I usually draw more women.
Also, as a random side note, the woman in the top left of the picture below with the smug look on her face is actually based on a rather annoying and particular lady that I had to deal with while working at the local library. To make matters worse, she had a nauseatingly high pitched voice that was so nasal it made you gag a little when you heard it. Needless to say, you should never irritate an illustrator if you don't want to be depicted in a cartoon later on.