Driving is generally the easiest way to get around, with a few exceptions. Toronto has excellent public transportation, and a car isn't necessary to get around downtown. Chautauqua is largely a no-car zone, but there are parking lots near the main gates. In Niagara Falls most major sights are clustered in a walkable area around the falls, and you should not have any trouble parking in Niagara Falls or Buffalo. But asides from these exceptions, towns and cities aren't easily traversed by public transportation.
Access to western New York from the east and south is primarily via Interstate 90, the New York State Thruway. Getting to Niagara Falls, Ontario, from Niagara Falls, New York, is a 10-minute drive when borders are clear. Some neighborhoods in downtown Buffalo (Elmwood, Allentown) are easy to explore on foot; traveling from neighborhood to neighborhood is best by car, however. Transport between Buffalo and Toronto by bus and train is frequent and convenient; getting around Toronto by subway and streetcar is preferred. Note, however, that border wait times on a bus or train are much longer than if you're traveling by car, since every passenger must be processed.