Can I do Niagara Falls as a day trip from NYC?

Yes, you can, but it's a brutal day. You're looking at a 7-hour bus or train ride each way, so that's 14 hours of travel for maybe 4-5 hours at the falls. The cheapest option is taking a Megabus or FlixBus from NYC to Buffalo for around $30-50 round trip if you book early, then you'll need a local bus or Uber to get to the falls. Amtrak's Maple Leaf train is more comfortable but costs $100+ and still takes 8 hours. Realistically, you'll arrive by 1-2 PM and have to leave by 7 PM to get back to NYC by 2 AM.

Honestly, unless you're desperate to check it off a list, I'd say no. You'll spend more time in transit than actually enjoying the falls, and you'll miss the best parts like the evening illumination or the fireworks (which only run certain nights). If you do it, skip Maid of the Mist (long line, rushed) and just walk the American side and cross the Rainbow Bridge for the Canadian view. Bring snacks because the food on both sides is overpriced tourist crap. If you can swing it, fly into Buffalo (1.5 hours from NYC) for $100-200 and save your sanity.