Avocado: An excellent source of monosaturated fat, vitamin K, folate and vitamin C. It's also very creamy and adds a nice richness to your smoothie. Just be aware that avocados are also very high in calories, so use them in smaller amounts.
My suggested combo: avocado, banana, strawberry, plain yogurt
Cucumber: Also very subtle in taste, they don't add much fibre but they do add phosphorus, manganese, potassium and vitamin K. They are also very low in calories.
My suggested combo: cucumber, pineapple, strawberry, lime, ice
Sweet Potato: Cook up a sweet potato (peeled) and mash it up. Add it to a smoothie for a very tasty experience. Sweet potato is bursting with vitamin A and fibre plus it adds a great creamy texture to the smoothie.
My suggested combo: sweet potato, apple, banana, plain yogurt, cardamom, cinnamon
Greek Yogurt: Use Greek yogurt in place of regular yogurt or milk to give yourself a boost of protein. The great thing about Greek yogurt is that even the low fat version is super thick compared to low fat regular yogurt, so you get all the creaminess without all the saturated fat!
Ground Flax Seed: An excellent source of omega 3's and soluble fibre which are both great for cholesterol. You shouldn't need more then about 2 tbsp.
I'm not a big advocate for smoothies because I often find people load them up with too much added sugar or drink too large of a portion. A one cup serving is ideal and if you've added in the right balance of high fibre fruits and veggies and a source of protein (like Greek yogurt or protein powder), that should be more than filling.