I lived in T.O for 5 years, but grew up in western Canada and now live in San Francisco, California. Toronto is not a tourist town like NYC is, but the most liveable big city in North America and probably the most ethnically diverse. Crime is low. You feel safe in most parts of the city. Wheather sucks in mid-winter(cold, but little snow) and sucks in mid-summer(hot and humid).
Most of the life/activity is in the "downtown area" during the day and at night. Cabs are affordable and transit/subway is good.
Anyhow if you are a tourist try these thing:
Amusement Parks:
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Paramount Wonderland (lots of big rollercoasters)
Ontario Place (water rides)
Centre Island Amusement park (for toddlers)
Playdium (video game heaven.. or hell)
Buildings:
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CN Tower
Casa Loma
Eaton Centre
Nightlife:
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Richmond & John area (aka Entertainment District)
Brunswick House (for drunken college kids)
Yorkville area (a bit more upscale)
Pubs:
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Madison Pub
Pauper Pub
There are so many large sub-areas in Toronto: Chinatown along Spadina, Greektown along the Danforth, Jewish area in Thornhill, Little Italy(and Portugese area), Little India(gerrard area), Artys(Annex & Queen West), Yuppies(Beaches), Gays(Church st.)
The music scene in Toronto is prety good. Lots of rock concerts happening and DJ culture is fairly mainstream there too.