If you don't want to use fruit in your smoothies, you can try a few ways to improve the taste.:
add half of an avocado to ake your smoothies creamier
sprinkle some cinnamon
add mint or basil: they have a lot of flavors naturally
squeeze half of a lemon in your smoothie: it will cut down on the bitterness of the greens
add a drop or two of stevia to sweeten it up