You could do baby spinach, bananas, and strawberries. If you want a sweet and frozen treat, you could use either frozen bananas or frozen strawberries in there. You can really use any kind of tender green. Any kind of lettuce is good. Stay away from Kale and bitter greens--they're horrible in smoothies, at least in my opinion.
I also think that any good smoothie must have at least some banana. It makes it really creamy. (:
But you can use any kind of fruit that you'd like! I hope that this helps!