I work in Toronto's so called downtown underground or rather the 'PATH'. I cannot fathom why I have found so many tourists wandering around lost in it's corridors. The idea of miles of underground walkways filled with shops connected all together by tunnels may seem fascinating to visit but actually they really are not that interesting. These small malls and their shops were designed as a convenience for the employees who work in the office towers that are located overtop of them so that they do not have to go outside in the cold Canadian winter. The shops themselves are not worth going out of your way to visit after all you have the Gap and HMV in virtually every other major city in world. The only site that might warrant a visit in the underground is The Hockey Hall and Fame and Museum which is located in the spectacular BCE Place building.
Fun Alternatives: Go to a real mall if you want to shop. If you are downtown, then go to the Eaton Centre. If you are staying in the suburbs check out the Yorkdale Mall.
Many tourist will be eager to find out what this 'Path' is. Around the downtown core, you'll find signs that lead to the PATH. What the path is, it's a virtual underground city. As a matter of fact, it's the largest underground path in the world. It's filled with stores, food courts upon food courts, and just about anything that you can find is in the path. The trap? It's HUGE and you could easily get lost. My suggestion is, if you really want to explore the path, either A) Travel with a Local that knows it, B) Get a map from the hotel or travel info office or C) Expect to get lost and plan accordingly
There are many exits to the streets so it's not as if you will can't get out. The problem is, once you get out, do you know where you are? :)
Perhaps i'm wrong and you can master the path, i dare you to!