Quoted from roar:taking the Horn Blower/Maid of the Mist tour is a must if you haven’t done it before and don’t plan to be back, it will get you so close to the power of Mother Nature it is amazing.
I would recommend this as well. Something about being below the falls and being confronted with something so large is pretty amazing.
roar posted some good Toronto stuff. It's all in the same general area (downtown) so any of those could happen during your 1 night.