These right brain/left brain exercises by Andrew and Mary Bragg are from the ThinkingManagers.com website.
Although too much has probably been made of the right brain/left brain differences, it is interesting to consider which of the activities on their lists you tend to do as a matter of course.
Due to the nature of my work, I read a lot of nonfiction. As a result, I've found that including books that are either completely outside of my usual routine or are flat-out bizarre (e.g., Neverwhere by Neil Gaiman) has helped to expand my creativity.
As an exercise, it resembles the old Spanish anarchist proverb: "If they give you lined paper, write against the lines."