It's a big place -- southern Ontario is larger than England, Scotland and Wales combined. If you thought Texas was big -- wait until you see Ontario. The recommended websites are great -- and Toronto is a world class city with lots to see and do. (e.g. Royal Ontario Museum, CN Tower, Ontario Place, Ontario Science Centre, St. Lawrence Market, Chinatown, Theatre District, The Danforth, Art Gallery of Ontario, SkyDome with the Toronto Blue Jays and the Toronto Argonauts, Queen St. W., Yonge St., Bloor St., Hockey Hall of Fame, Canada's Wonderland, to name but a few places in the Big Smoke.)
Niagara is worth a vacation of its own with the Falls (just the Falls -- I avoid Clifton Hill at all costs); the town of Niagara-on-the-Lake has Ft. George National Historic Site (War of 1812 fort where you can learn that Great Britain won the War of 1812) and the Shaw Festival, as well as lots of expensive stores. There are about 50 wineries in the Niagara Region, many of them with wine-tasting and the opportunity to learn more about the art of winemaking in southern Ontario. The Niagara Escarpment is a world-rated ecological zone stretching from Niagara Falls to the tip of the Bruce Peninsula (separating Lake Huron and Georgian Bay -- the sixth Great Lake). On the way from Niagara to Toronto, you could visit the Canadian Warplane Heritage Museum just outside the city of Hamilton.
Ontario -- yours to discover -- it's so much more than Niagara Falls and Canada's Wonderland.