Interestingly, since the Canada side of the falls is so close to the US, if you're staying in the big hotels along the river you'll likely be connected to US towers much of the time. At least it was that way for me on my Droid 3 two years ago.
Go into settings / more / mobile networks and verify that "Global data roaming" is unchecked. That will prevent data over mobile on Canadian carriers, which would be the most expensive problem.
I can't remember what voice minute or messages will cost in Canada, but I do know that Verizon has a Canada/Mexico plan for $15 that gives you 1000 minutes. I am pretty sure that you can activate that feature at will, so pay for a month and then cancel, if you know you'll be making calls and sending and receiving messages that would exceed $15 a la carte (I just looked it up - $0.89 per minute voice, I think texting is based on your plan in Canada.) But, again - if you're close to town, the falls, and the river, you may find yourself connected to verizon towers on the US side.
Unfortunately, at the moment I'm traveling and can't access what I paid Verizon after our trip, but I don't think it was wildly extra. When I get home next week I'll pull out my old bill and see how much extra we paid. (The day that we left my daughter had a car accident so I was on the phone quite a bit as we were checking out and driving home to call the insurance company, call her, etc.)