NoahGrey, you can use any other juice, really. I have made smoothies with a million different juices. Some of the new carrot & berry juices are quite good. Best use something without a lot of added sugar, though, or the smoothie is tooooo sweet.
My dog, Connor, loves these smoothies too! I've often made ice cubes out of them for him for a summer treat.
Our favorite ingredients are strawberries, banana, and orange juice, with a spoonful of vanilla or strawberry yogurt. I agree with someone's above post - the plain yogurt isn't so good in them!