The Peace Bridge charges toll only into Canada--it's free leaving Canada!
If you're headed for downtown Toronto, fly to YYZ & take a bus/taxi. Having a car is a nuisance between the rental costs & parking fees/problems--just like Manhattan, NYC. If you also want to see suburbs, N. Fls, etc., consider an open jaw: home to YYZ, pickup a rental car when you're about to leave Downtown, fly home from BUF or even DTW (to which Hertz USED TO BE one-way friendly.)
Finally, please note that Canada Customs will not allow Canadian citzens to drive a US-plated rental car into Canada: "Illegal importation of a foreign car."

Yankees & other foreigners--no problem.