Watkins Glen would be a great stopover for you, and it is just over halfway to Niagara Falls, though only a 4-5 hour drive. Here is a guide for some things to see in that area, and another for Niagara Falls.

I'd recommend driving the southern route through Pennsylvania on I-280/I-80/I-81/I-390/I-90. You can map it on Mapquest or Google Maps. This route keeps you on interstate highways most of the way, but it is more scenic than the New York Thruway (I-90). If you have the time and want to see another good park, Letchworth State Park is along the way on I-390 at Mount Morris Exit 6. You can spend a couple of hours in there.

Don't forget to take passports with you to cross the border into Canada, as there is lots to see on both sides at Niagara.