Nuts and seeds are a popular addition to smoothies, because they add flavor and protein. Check out my peanut butter banana smoothie recipe (I like to call it the "Elvis special" for a great way to use peanut butter or any kind of nut butter in a smoothie).
For a sweet addition to smoothies to make them more dessert-like, try chocolate chips or white chocolate chips. They would make a nice addition to my chocolate cherry smoothie recipe. It already tastes like chocolate covered cherries. The addition of chocolate chips makes it that much better. Try all of these ideas, and let me know what you think.
Strawberry Smoothie image by James Baigrie/Getty Images 2013, licensed to About.com Inc.