I bought a 300g/0.66 lb. bag of frozen mixed berries (strawberries, raspberries, blackberries, blueberries). Anything in that ballpark size-wise will work.
Feel free to use any kind of apple you wish. I like the Red Delicious ones because they tend to be sweeter than other varieties. It also means this smoothie doesn't need any extra sweetener.
Serves 1-2 depending on how much you drink.