You cannot go wrong with a smoothie. Ever. It's creamy and fruity and you can hide good-for-you things in them and no one will ever be the wiser, unless, for some reason, you let slip that you put whey protein in when they weren't looking. Don't do that, though. Seriously. Just put things in and serve that smoothie up and grin when they can't quite place everything residing in the frozen delicacy.
There are even cheat sheets to make these heavenly creatures, even though, let's face it, they're pretty hard to mess up.