Some people actually feel old when they turn 30 so an over the hill might not be the best idea. I could be being too picky though. Adult-oriented cakes can be fun but may offend people.
An idea might to find out some of her childhood interests and put a design that represents something like her favorite cartoon or toy. You can also do a timeline sort of idea where you mark down important events throughout her life. Another idea would be to leave the cake blank, with just a simple frosting, then have her friends and loved ones sign it with different colored frosting. Those tubes of frosting you can buy can act as pens.