So I recommend working on adding vegetables into your smoothies along with some healthy fats and protein to help keep your blood sugar more stable.
If you are new to adding vegetables than you will want to start with some mild flavors such as spinach, zucchini, and cauliflower so that you get the added nutrients without changing the flavor profile of the smoothie. I also love to add avocado to mine which creates a thicker consistency and provides me with some healthy fat to help keep me more satiated.
Once you start adding more vegetables you can get more adventurous and start adding celery, carrots, romaine lettuce, swiss chard, and herbs as well. Start with you favorite smoothie and test one additional flavor to see how it works. If you love a strawberry smoothie, maybe add some spinach and basil to it. Blueberry a favorite, try adding some fresh thyme and cauliflower. You won't always get a winning combination, but once you begin to practice you will find some favorites to enjoy.
One last tip, go through your veggie drawer each week and whatever vegetables are becoming a little sad that you won't use in a quiche or a soup; chop and freeze for your future smoothies. You can also chop up greens and filtered water in a blender and make ice cubes to save for future smoothies as well.