Bananas turn up again and again in smoothie recipes because they make the blend smooth, creamy, thick and sweet. They are also the key ingredient in many recipes, giving the smoothies their substance. To replace them, you'll need ingredients that supply all these qualities, or your smoothie will turn out grainy, runny or bland.
Adding Sweet Fruits
If you aren't avoiding bananas because you're reducing the amount of sugar in the recipe, replace them with a different fruit. Choose one or two that contain low to moderate amounts of juice -- adding an orange or a handful of grapes contributes more liquid than solid and won't give you the creamy texture you would get with a banana. Instead, choose mango, peach, persimmon or berries. To make the smoothie even thicker, use frozen fruit instead of fresh.
For a thick, creamy smoothie without the sugar of extra fruit, add a source of healthful fat instead. Avocado, coconut meat or Greek yogurt can contribute the silkiness of a banana and also give the smoothie some substance, so you aren't hungry again an hour after you drink it. A small quantity of these can go a long way, so add fats gradually until you find the ratio that satisfies you. For example, start by blending in 1/4 of an avocado -- you might find that it is enough. If not, add up to a whole fruit depending on your needs and tastes.
Thickening agents can compensate for the banana, and options go far beyond cornstarch. Oat flour or oatmeal that you have powdered in a spice grinder or coffee grinder adds bulk to smoothies -- The Kitchn suggests adding about 1/4 cup -- and chia seeds absorb water to form a gel that lends thickness to any blend. Soak the seeds in advance for a more substantial gel, or add them dry to the smoothie and let them sit for a few minutes before blending. Frozen vegetables also contribute thickness; try spinach for its mild flavor.
One of the selling points of bananas is their natural sweetness. If you're happy with an unsweetened smoothie, there's no need to compensate, but to retain the taste profile of the original recipe, you'll need to add a source of sugar if you've replaced the banana with anything other than fruit. Honey, agave syrup and maple syrup are all thick, smooth sweeteners, and they're all sweeter than table sugar, meaning you can use a smaller quantity of each to get the same result.