The great thing about smoothies is that anything goes.
Whatever you have on hand and like as a combination is good. No need for anything fancy or a quick last minute run to the store because you are missing a key ingredient, just go with what you have.
Refer to this previous post for basis of Green Smoothie making.
Green Smoothie #2:
2 handful curly kale
1/2 tsp spirulina
1 Tbsp wheatgrass powder
2 C water
I like mine with a touch of spicy, hence the cayenne, which is optional of course.