Need help with itinerary From Toronto to NYC in 3-4 days (mid-April)

Hi,

We're a couple in our thirties traveling on a budget, coming to North America from overseas. We're visiting Toronto for a couple of days and then want to head to NYC (where we have a flight back home). Since we really enjoy nature, we'd like to see if it's possible to take 3-4 days just for the trip from Toronto to NYC in scenic areas.

For this we will rent a car, but from what I understand, it's not always possible to take a rented car from CA to the US, so the best option would be to travel to Niagara Falls by public transport, then catch a bus to Buffalo airport and rent a car inside the US (correct me if I'm wrong and there's a better way to do it....)

Here come the questions:
Can you suggest a nice itinerary for a 2-3 day road trip from Buffalo to NYC? Would 2-3 days be enough to enjoy it?
And would it be enjoyable in mid-April? (no option to change the dates...)

We've never traveled in the US before, so any info would be highly appreciated!
Thanks :)

Yulia.