Feel free to try any other leafy green! If you choose lettuce, choose a darker one like Romaine or red leaf lettuce vs. iceberg. The darker the color, the better!
Also, you may not like spinach by itself, but often including in a Blast, the flavor is masked by the other ingredients. Banana, pineapple, cinnamon, and cacao are a few that help mask the greens flavor. If you haven't already, I'd suggest you give it a try :)
Here are some other leafy greens to choose from: