Teenagers are notoriously hard to please, but it's not impossible to find a birthday cake idea a teen will enjoy. The thing to keep in mind is to always customize the birthday cake according to what the teenager likes, while trying to not embarrass her. If nothing the bakery is offering looks good, think about your teen's hobbies and tastes to come up with a birthday cake that will enhance her party.
Photo cakes are very popular and a teen might enjoy a cake with his image on it. Alternately, you can take a picture of your teen with his favorite pet or friend. Take the picture to a bakery that can superimpose the photo on the cake.
If your teenage girl likes shoes, you can decorate the cake with tiny plastic shoes. For a teenage boy who plays baseball, the cake can be decorated with tiny bats and balls. Craft supply stores will have any decoration you can't find at the bakery.
Buying a tiered cake (as opposed to a flat sheet cake) may impress the hard-to-impress teenager. Three-tiered cakes decorated in the teen's favorite colors and filled with the teen's favorite topping are one route to go. Most bakeries can custom-make tiered birthday cakes and may have decoration ideas.
If your teen likes a certain band, TV show, movie or celebrity, you can create a cake themed around this. For example, your cake can depict a certain scene from a movie. Get the tiny "set" decorations at hobby stores that sell dollhouse furniture. Musicians, TV shows and some celebrities have websites where you can download pictures or small movie posters. Superimpose these on the cake.