Liquid level marks, like a measuring cup, for the inside of your toilet.
Think about how much time you'll save not having to pee in a measuring cup before dumping it in a toilet! So much time to gather other metrics about your life
It would be off the waterline of the individual toilet. I imagine the level indicators would need to vary from toilet to toilet because of different toilet bowl shapes.
This could create a bridge between those companies making fly stickers for urinals and the big toilet corporations.