If she eats them, raw eggs or coddled eggs mixed in are another good addition, likewise heavy cream, ice cream, puddings.
Apart from shakes, you can do a lot with soup in the blender, too, adding pureed meats if applicable and a lot of the same ingredients above.
I would recommend real food ahead of powdered ingredients as the latter take more energy to metabolize, can get clumpy, and all in all aren't generally as tasty. And just a general note that given "a spoonful of sugar makes the medicine go down," we are no longer as worried about long-term heart disease, etc. so however much honey, salt, chocolate, maple syrup, or other flavoring improves drinkability for shakes, soups, etc. and doesn't have GI consequences, go for it.