In trying to locate an old musical fav app of mine (Magic Fingers, odd name but cause and effect with classical music, free), I found a cute early app for visualstimulation and cause and effect from Finland called Onni and Ilona Animals for IOS. 3 animals are available in the free version and all 12 cost $3.99. Very nice color contrast, not too visually busy, great for visual tracking, and early use of hands or a switch.
Here is a video from the company that does not show the baby intersecting with the app, but a cutie nonetheless.