First time there? Do the tourist stuff.
Admittedly, it's not my favorite Canadian city, but I spent three days there a couple years ago. I took the bus/subway from the airport to downtown, where I was staying, and walked over to Queen St. for some dinner.
Next morning, I took a ferry out to the islands, walked around a bit, and enjoyed the view of the skyline.
Explored the ethnic neighborhoods, of which there are tons, and checked out the skyscrapers in the financial district, Union Station, and so on.
Went to the CN Tower around sunset and watched the lights snap on over the city.
Next day, took in a few more sights, Maple Leaf Gardens, the historic district just east of downtown with the covered markets, Chinatown, etc.