The famous Niagara Falls are located about 2 hours by train from Toronto, as you say it is a must when you come out for a few days!
A little geography first: the "Niagara Falls" are in the middle of the Niagara River that connects Lake Ontario to Lake Erie. The Niagara River forms the border between Canada and the United States of America.
These are the most powerful waterfalls in North America at a rate of more than 2800 m3 / s, a priori there are three separate falls, but to be quite honest I've only seen two (American Falls and Horseshoe Falls) ...
We've been enjoying the falls twice, the first time in the company of Jannick in November 2012 (and it was cold and we were almost alone on the site) and the second in the month of April 2013 with the family of Benedict (and it was hot and there were lots of tourists like us)
I advise you to leave quite early if you want to enjoy your day on site. From Toronto around 8 am, you can take during breakfast 'on the train, it's not too expensive and it's pretty good.
Arrived at the station in Niagara, the easiest way is probably to take a taxi to the site of the falls itself. It takes about $ 15 for the trip, but it will save you a good walk. Once there, there has to open wide their eyes and enjoy!
