You can walk across the border from Niagara Falls, NY to Niagara Falls, ON. It take about 15 mins or so. You do have to have a passport to re enter the USA.
On either side take the Maid of The Mist ride. It is AWESOME!
Where are you staying in Niagara Falls, NY? There is an Aquarium within walking distance of the Falls, and I've heard it's good value for the price.
I have friends who live in NF,NY and I will let you know if they have any other suggestions...
Theresa