I used to work at a juice stand in high school making different flavors of drinks. We used two bases for the drinks, either apples or carrots as the major "juice" ingredients. Both provide a good taste. With a juicer, I suggest you put whatever you want through first, then the apple or carrot in last, as they clean and push the other ingredients through with their higher water content.
To change just juice to smoothie I'd suggest you find something with enough water to support what you are doing. "Smoothie" implies it is smooth meaning thicker then just juice, but slides along the tongue and palate. You are going to have a harder time finding veggies that can do that without having a fibrous texture.
Some we used to do (keep in mind all juice):
Apple, carrot. The king of easy.
Carrot, apple, parsley, ginger.
Carrot, celery, broccoli, spinach, beet.
Apple, orange (these two juiced) blueberry, strawberry , banana (blended with juice).
Possibilities are endless.