However, recently I've been using plain kefir to create smoothies and loving the results. The best combinations so far are raspberry-lime and orange-peach.
I created a raspberry-lime version with 1 cup frozen raspberries, 3/4 cup plain kefir and the zest and juice of one lime. This one is definitely on the tart side, so if you prefer a sweeter smoothie, I recommend adding only the juice from half of a lime.
For the orange-peach version I added one orange (peeled and chopped), 1 cup frozen peaches and 3/4 cup plain kefir. This is Chris' favorite and one that he now requests. Each version makes a 12 ounce smoothie with 8 grams of protein.