I am a huge fun of dumb and fun ideas, though I often lose interest halfway through. My most top-of-mind dumb machine vision idea that I'll never probably make is a "dog detection system".
Basically, set up a series of cameras across several rooms with a basic vision model so that you can always detect (via a local web portal or something) where a dog is within the house. This is especially useful in the case of a dog who eats shoes.
You could probably add some fun audio triggers for when the dog does certain actions or gets too close to particular objects if you add a speaker to each camera (for example if the dog gets too close to a detected pair of shoes or a nice lamp).
Then you could add some fun analytics for where the dog spends its time within the house, etc, etc, extrapolate a bit too much about the dog's psyche, preferences, and whatnot.