If you'd like to show who you are through the things that have influenced you, discuss a person, book, film or play that has affected you the most. A book that has influenced the way you think or your personal philosophy will be a great essay if you plan to major in English literature. Likewise, an essay on films works well for potential film majors and plays for potential theater majors.
Maybe you spent a summer dedicated to helping orphans in Mexico or you climbed Mount Kilimanjaro with your dad two years ago. Whatever you happened to have done, discuss how this experience has shaped you as a person. What did it teach you that another experience could not? How has this experience helped better you as a person, and why will it make you a better candidate to go to the college of your choice?
Trying experiences can both teach people a lot about life and build character. If you spent your sophomore year of high school undergoing treatment for leukemia or you helped care for your grandmother as she suffered from Alzheimer's, write about it. These kinds of experiences may be difficult, but they will show the admissions committees that you have the commitment level and emotional strength to get through a very trying experience.
Maybe you've never experienced extreme adversity or done something out of the ordinary, like spend an entire summer driving across the country. That's OK. Instead, write about an experience that taught you something about yourself or your family. An essay about your dog, the summer you spent folding T-shirts at a local apparel store in the mall or the best holiday you've ever had can be just as touching and stand out, as long as it is well-written and focused.