Niagara Falls is 130 km from downtown Toronto — about 90 minutes without traffic, 2 hours with.
GO Train + bus is the cheapest option (under $40 round-trip) and reliable, but the schedule is sparse and you arrive at a bus terminal 20 minutes' walk from the falls.
Megabus / FlixBus is cheap but slow on Fridays and Sundays.
Rental car gives you total flexibility but parking near the falls is $25–$40 and the QEW backs up most summer afternoons.
Guided day tour is what we do, obviously: hotel pickup, no parking, expert guide, two viewpoints, the boat, a winery, and Niagara-on-the-Lake — all in 9 to 10 hours. For most international visitors with one shot at Niagara, the time saved and the context added are worth the difference.