Don't know about ice cream, but adding mixed frozen cauliflower to chocolate milkshake is invisible to the taste for me (and I am a picky eater) and I find it much more filling. Some spinach work too but you can add too much before noticing it by taste/smell Even at a third, cauliflower is still invisible to me in comparison.

"Lol nobody can spy me, I put tape on the camera". Quote from my mother...