CN Tower

For 32 years, Toronto's iconic CN Tower held the record for the world's tallest freestanding structure. Though it has been eclipsed by buildings in Dubai and Shanghai, "the needle," as it is affectionately called by locals, still draws more than a million tourists every year. While there are other things to do at the CN Tower—such as eat in one of the restaurants, shop, watch a movie or do the EdgeWalk—just going up to one of the viewing decks is well worth the price of admission. Visitors are whisked up small, glass-enclosed elevators to the LookOut viewing platform (at 346 m), or to the SkyPod (at 447 m).
Both the viewing platforms give you a 360 degree view of Toronto and Lake Ontario. From the higher SkyPod, you can see up to 160 km on a clear day—all the way to Niagara Falls. Though it may have lost the record for tallest structure, the CN Tower still holds a few records, including world's highest wine cellar. You can top off your visit with dinner and wine at the revolving 360 Restaurant, and enjoy some of Toronto's finest cuisine while you savour the view from the top.

Bonus: Ripley's Aquarium is the newest addition to the plaza at the base of the CN Tower and should not be missed.