I was a bit surprised that the one-way rental on a car between Niagara Falls and NYC was only about $220(US). vs. a minimum of about $570 for picking it up in Toronto and having it for an extra day. I tend to agree with Michael that bussing to the Canadian side of the falls, walking across one of the bridges, picking up your rental on the American side, then driving back to pick up all your gear is the way to go. There's an Enterprise franchise that's off-airport (saving a bit) about 6 miles from the Rainbow Bridge (just a shade less than the distance to the airport), so the cost of a taxi/Uber/Lyft should not be prohibitive. You will need to do some careful shopping given the number of variables involved in your car rental, including one-way, pick-up location, type of car, geographic restrictions, your age, etc., but it should be possible.

What I'd really like to address is some of the many wonderful natural attractions that fall more or less right along the route you'll be following. First up would be Letchworth State Park near Geneseo, then Watkins Glen State Park, then since you're going to be near Ithaca for Robert H. Treman State Park I'd strongly suggest (even picking it ahead of Treman) Taughannock Falls State Park where the falls, although not as spectacular as, are higher than, Niagara and the roughly mile-long back into them is a delight. All of those would fit in with a 'Waterfall RoadTrip' and one last one, Kaaterskill Falls before you finish up in NYC.

AZBuck