Niagara Falls
What can I say, it's pretty cool. And yes, the view is prettier from the Canadian side. Don't try to take mace or pepper spray over the border either; my mom had to surrender her key chain (which was a non-functioning mace sprayer that was probably 20 years old) 'to the crown' as the form said.

I'm sure the Maid of the Mist is pretty cool, but we went down to the rocky cliffs on the American side under Bridal Veil Falls and you can walk under the water and get pretty wet if you want.