This is such a simple and common sense idea, I can't believe I haven't seen it before. This is the toilet at Chipotle, where I went for lunch today. There are two flushing options, the green button that uses a small amount of water and the silver one that gives a bigger, longer flush. (I only needed the green one, don't worry.)
The only odd thing was that there was motion sensor and it auto-flushed, so I didn't get the chance to choose a button. Get that little wrinkle fixed and this will be a super improvement. Saving the planet (and their water bill) one flush at a time.