2 solid days + 2 travel days in Niagara Falls? That's a lot of time in my opinion. I went to University in Buffalo and there's more to see than just the Falls. Buffalo itself can be a lot of fun, Niagara on the lake is rather beautiful & Toronto is always a lot of fun.
You may want to spend some time spread over 2 or even 3 areas. The falls to me is a 1 day experience. Getting downtown from the Airport is very very easy and can be very cheap. Some hotels offer free shuttles and there is a cheap city express bus.

As for car rentals there are come memberships that remove the high rental costs such as USAA if either of you have family in the military.