I've been asked recently by a friend of mine in the United States what do they need to enter into Canada at Niagara.
Here's the simple answer...you need a U.S. Passport. Children under 15 travelling with both parents need to carry a U.S. birth certificate (the originals).
Here's what generally happens when you reach the Canadian end of the Rainbow (offers a dramatic view of the falls as you enter Canada), Peace or Lewiston-Queenston Bridges. All border entry points to Niagara.
A border screening agent will ask some of the following questions, "Where do you live? You are a citizen of what country? What is the purpose of your trip? How long will you be staying? Are you bringing anything to leave? Are you bringing in any firearms, tobacco or alcohol?
Entering Canada from the U.S. can be simple if you answer the questions honestly.
I've attached a photo of the Rainbow Bridge at Niagara Falls and a view of the falls you see from the bridge.