Cut up overripe bananas into 1/2 inch pieces and put them in a Tupperware dish and keep in the freezer. When it's time for a shake, just drop in a few frozen banana slices into the blender along with a cup of regular milk or almond milk, your protein powder, and blend. Tastes like a banana milkshake and it's simple and fast. You could also add a tablespoon of ground flax seed for fiber if you like.