One of the most famous attractions in all of North America, Niagara Falls has built its reputation on having the greatest combination of water flow and height of any waterfall in the world. Shared between the United States and Canada, the larger percentage of the falls are on the American side, while the best views are definitely had from Queen Victoria Park on the Canadian side. If you're visiting from Toronto, it may benefit you ride for two hours on a mass transit bus like Megabus (under $20 USD one-way) or the more comfortable VIA Rail (about $60 USD round trip) as a day trip from the city on the cheap. If you're looking to cross to the American side, you will need valid identification and a passport, although you'll find that your fill in entertainment and enjoyment of the spectacle can be had in Canada without dealing with border crossing.
With the falls comes a massive tourist industry, whether it is falls-related (the hugely touristy Maid of the Mist, a boat that puts you into the eternal spray at the foot of the falls, and Skyon Tower, a Space Needle-like observation deck and restaurant with excellent falls views) or any of a number of entertainment options that include indoor water parks, casinos, Marineland, and tons more. Most popular during the summer, when chances are highest that you'll be jostled by the digital camera-wielding tourist crowd at every turn, you'll find that winter is just as opportune a time to take in this natural wonder. The falls will still be at high volume and the ice floes and snowy landscape could be the basis of a bundled-up, but equally rewarding adventure.
Note:Pedestrians and bicyclists can pass pretty easily between the Canadian and American sides of Niagara Falls via the Rainbow Bridge, which costs fifty cents each way and comes with stellar views of the waterfalls.