We visited Toronto last month (via YYZ from the U.S.) and then took the train to Niagara, ONT to visit prior to driving to Cooperstown, NY. I was unable to find a single major rental company or itinerary that would let me rent in Toronto or Niagara and return to an NY location (Utica, Syracuse, Albany). I know it's allowed, I just could not find anything in reality. So we ended up taking a cab across to Niagara, NY and renting there (we tried two Ubers, neither could or would take us across the border. It was pouring rain or we would have just walked across to the NY side. Would have been cheaper!!). We got the car and drove back across to the Canadian side where we discovered we should have done another ArriveCAN to cross back in via land - but the border agent admonished us and let us cross anyway. So just a heads up if you're a Yank visiting and planning to go back and forth make sure you complete your ArriveCAN each time.