My wife and I, citizens of Italy, will be travelling to New England for sightseeing. We'll have an ESTA (US visa waiver). We will arrive and leave at/from New York JFK and rent a car. 

Since we would like to see Niagara Falls, we will need to cross the US-Canadian border and come back on the next day or so. 

Should we expect any trouble obtaining a Canadian visa at the border, or reentering the US afterwards?

Can we take the rental car across, or do we need to clear it with the rental company?