Really depends on the individual/group and their personal interests but here's my generalized list of suggestions. Not necessarily in absolute order:
1) CN Tower
2) Toronto Islands (seasonal)
3) ROM
4) Casa Loma
5) Distillery District
6) AGO
7) St. Lawrence Market
8) Ripley's Aquarium
9) City Hall/Nathan Phillips Square
10) Eaton Centre
Additional options:
-Parks: High Park, Riverdale Park, Trinity Bellwoods, waterfront trail all the way to the Humber Bay area, Scarborough Bluffs, Rouge Park, etc
-U of T downtown campus
-Canada's Wonderland
-Toronto Zoo
-Historical theatres: Ed Mirvish theatre, Elgin and Winter Garden theatre, Royal Alexandra, Massey Hall, etc
-Historical buildings: Union Station, Royal York Hotel, Gooderham building, Osgoode Hall, Old City Hall, Queen's Park, St. Lawrence Hall, old downtown churches, etc
-Notable neighbourhoods: Kensington Market, St. Lawrence area, Financial District, Queen West, King West, Ossington, Chinatown, The Annex, Yorkville, The Beaches, etc
-Black Creek Pioneer Village
-Other museums: Ontario Science Centre, Hockey Hall of Fame, Bata Shoe Museum, Gardiner Museum, Fort York, Aga Khan, Spadina Museum, etc
-Sporting event: Raptors, Maple Leafs, Blue Jays, Toronto FC, etc
-Seasonal events: CNE, Pride, Caribana, TIFF, etc
-Malls: Yorkdale, Dufferin Mall, suburban malls, etc
-Restaurant/bar hopping areas: Queen West, King West, Ossington, Dundas West, Leslieville, Chinatown, Kensington Market, College Little Italy, The Annex, Koreatown, the Junction, Bloorcourt/Bloordale, Greektown, Midtown, NYCC, the suburbs, etc
-Out of town excursions within 3 hours or so: Niagara Falls, Niagara on the lake, Hamilton (waterfalls), Prince Edward County, a variety of nice small towns, provincial parks, etc
