The Niagara Falls are actually three water falls for the price of one. Goat Island separates the two major falls, the Horseshoe Falls which lie mostly on the Canadian side of the border and the American Falls on the US side of the border. The smaller Bridal Veil Falls are also on the American side and are separated from the main falls by Luna Island.
Just to confuse matters further, on both sides of the border you will find a city called Niagara Falls. The twin cities are connected by three bridges, with the Rainbow Bridge just downriver from the Falls. It can be used by both vehicles and pedestrians and not only affords great views of the Falls, but also allows you to view them from both sides of the border. And assuming you've come quite a distance to marvel at the Niagara Falls, who wouldn't want to see them from both Canada and the USA?
The best views of the Niagara Falls have traditionally been from the Canadian side. Queen Victoria Park, with its beautifully manicured gardens, has viewing platforms offering spectacular views of both the American and Horseshoe Falls, and there are also underground walkways there that into observation rooms which give the illusion of being within the falling waters.
The observation deck of the nearby Skylon Tower offers the highest overhead view of the Falls and you can also see as far as Toronto in the opposite direction. It is also home to a revolving restaurant with fantastic views. Along with the Minolta Tower, it is one of two towers in Canada with a view of the Niagara Falls, but an unfortunate side effect of the construction of tall buildings in Niagara Falls, Ontario, has been to create a layer of mist that often obscures the view.
From the US side, the American Falls can be viewed from walkways along Prospect Point Park, which also features an observation tower and a boat dock for the Maid Of The Mist, a boat tour of the Falls which has been in service since 1846. There are several Maid Of The Mist boats and it is possible to board one on either side of the border, but prepare to get wet! And consider a disposable waterproof camera rather than risk damaging your own if you do venture out on a boat.
Goat Island offers more views of the Falls and is accessible by foot and vehicle by bridge above the American Falls. From Goat Island, a lift takes you to the Cave Of The Winds and leads hikers to a point beneath the Bridal Veil Falls.
If you are spending a couple of days at the Niagara Falls consider buying a Great Gorge Adventure Pass. It includes the Maid Of The Mist, Journey Behind The Falls, White Water Walk, and the Butterfly Conservatory, a one-day People Mover pass and discounts for other attractions, but you don't need to do everything on the same day. One idea would be to to do the Maid Of The Mist and Journey Behind The Falls on one day, and use the one-day People Mover pass to do the White Water Walk, Butterfly Conservatory and Botanical Gardens plus the Aero Car (all of which are near each other, but further from the Falls) on the second day.

If time is against you, then take to the skies in a helicopter for a whirlwind not to mention breathtaking view of the Falls in all their glory. Niagara Helicopters have been flying over the Falls for 40 years now and you can book tickets online before you go. Prices for 2009 are £59 per adult and £39 for children aged 2 to 11.
The volume of water going over the Falls varies throughout the year, with the peak being during late Spring early Summer when twice the water witnessed during the Summer goes crashing over the Falls. In the winter and at night, the volume is halved again as water is diverted to providing hydro-electric power.