For the first (pretty long) leg, I can't offer much advice. Google maps says basically I-77 to US19 to I-79, which looks reasonable on paper but isn't a route I've ever traveled (that I can recall, at least).
For the Pittsburgh to Niagara Falls leg, I'd take the obvious I-77 to I-90; alternate routes are a good bit slower. The Thruway (I-90) does have tolls that add up; if you have or can get an EZPass transponder it makes life a little easier and often a bit less expensive, but paying in cash does work.
For the third leg, I-390 to I-99/US15 is good. Stopping at Letchworth State Park is definitely good if you can swing it; I would even suggest cutting out a day (or two) of Niagara Falls if that's needed to get one at Letchworth. Stopping at Knoebels in Elysburg, PA is also great if you're at all into amusement parks as it's one of the neatest ones anywhere.
Sounds like a nice trip. I hope you have a great time.
