You can't visit Toronto and not make the trip out to Niagara Falls. It is a natural wonder of the world. Besides the Falls themselves, the city offers a great array of tourist and entertainment destinations from casinos to a wax museum, while the region has some of the best vineyards in the country, the world famous Bruce Trail, an array of historical sites from the War of 1812, and is very picturesque. Many companies offer bus tours of the region or its about an hour and a half drive if you have access to a car. VIA also offers train service to the region (GO does as well, just only from June-September).
Within the city of Toronto, if the weather is nice, go for an afternoon stroll at the Toronto Islands, High Park, Rouge (soon-to-be National) Park, Ashbridge's Bay, or the University of Toronto's Downtown Campus.
Toronto is a city of neighbourhoods and the best thing to do is really just explore the city. Each part of the city can be so different than another, not just in character, but also in built form. The St. Lawrence Market area (and the market itself), the Distillery District, Queen West, Kensington Market, the Fashion District, the Beaches, the Entertainment District (Toronto has the 3rd largest English-speaking theatre scene in the world, behind New York and London), Yorkville, even the Financial District are all worth paying a visit to. In addition to those, the city just about every kind of ethnic neighbourhoods you can think of. Greek, Italian (times two or three), Korean, Vietnamese, Jamaican, Jewish, Ukrainian, Polish, Portuguese, Chinese (times two or three), etc. Some of those neighbourhoods are a little more out of the way, so it may not be worth a visit.
In terms of attractions, the obvious ones have already been mentioned. The ROM is the largest Museum in the country, while the AGO is the largest Art Gallery. The Toronto Zoo, the Ontario Science Centre, Fort York, Casa Loma, City Hall, the Ontario Legislature, Union Station, and Yonge-Dundas Square are also popular among tourists. As a hockey fan, you might also want to put Maple Leaf Gardens on the list.
Walking or the TTC are the best ways to get around Downtown. Most attractions and neighbourhoods are on or near a subway stop or streetcar line. If you do have to drive, be aware that it is illegal pull up beside a stopped streetcar. Toronto is one of the few cities on the continent that has streetcar service not operating in dedicated right-of-ways so most visiting drivers don't have experience driving around them.
Enjoy your visit. I know many in the West dislike the city but often when they visit, they change their mind.