I grew up in Rochester, NY, which is only an hour away from Niagra Falls, so we went there usually about once a year or so. Both sides have a lot to offer, depending on what you are looking for.
The American side has viewing areas on Goat Island, that get you really close to the edge of the American Falls and Bridal Veil Falls. The "Cave of the Winds" tour brings you to viewing platforms at the base of the falls and gets you really close -about 20 feet. There is a small, but nice aquarium and a Daredevil museum. Both sides have Maid of the Mist tours, which is a "must do"! It is an incredible experience getting so close to the falls!
The Canadian side has a walkway with the best overall views of all the falls. They also have a tour called "Journey Behind the Falls" in which brings you through underground tunnels that having viewing areas behind Horseshoe Falls and to a viewing platform near the base of Horsehoe Falls. Really amazing! A block away from the falls is an area called "Clifton Hill", which depending on your tastes, is either a tourest trap, or the greatest selection of wax museums ever. Even though it is touresty, it is fun. There is "Ripley's Believe it or Not", "The Guiness Book of World Records", several haunted houses, etc. There is a Jungle themed restaurant which is interesting. Just be aware that to cross over to the Canadian side, you will now need passports. (This is new, since Homeland Security regulations kicked in.)
Here is a link which lists attractions on each side.
http://nyfalls.com/niagara/niagara-falls-vs.html