I do a lot of smoothies but I always use a lot of protein powder. If you can find a good tasting protein mix (there are several out there), you can really make some great smoothies. My favorite protein mixes are Champion and Dymatize(?). Banana is awesome. You can take the banana mix, put some peanut butter in there and a little hershey's syrup. Throw in some ice and you have a great and healthy meal. I also know a lot of people who will mix oatmeal into a smoothie / shake with their protein mixes to make a breakfast. Oh yeah! Lean Body has a breakfast mix that has 35 grams of protein, 40 grams of carbs and 7 grams of fiber. You can get them in blueberries and cream, bananas and cream and apple cinnamon. They are wonderful. They are also thick so you might want to add a little extra water to them. Also, a little tip. If you can mix it the night before and put them in the frig overnight, they taste wonderful in the morning.
Hope that helps!!