I of course couldn't possibly visit Canada and the USA without going to see Niagara Falls!
You need to be aware that this is a very touristy place and everything here (or almost everything) is designed to get you to part with your money. Visiting early in the morning in winter meant I was able to find free parking and see the falls completely frozen (well, almost)!
If you enjoy experiencing a sense of danger, go on one of the Niagara boat trips. Some of them take you almost to the very foot of the falls: an experience as unique and unusual as it is unforgettable. Consider wearing rainproof clothing so you don't get soaked however!
And finally, for shopping fans, there are some excellent outlets to be found in the vicinity of the falls!