I visited Niagra Falls while on a 2 week holiday in Toronto, and both the girlfriend and I drove from Toronto to Niagra. It's really the only way. It looks like it's no distance at all on a map, but you then realise you have to drive around the bit of water that's between Toronto and Niagra. 

As the others have said, rent a car, and leave plenty of time to drive there and get back. Traffic can be bad, it was for us when we came back to Toronto. Not entirely sure what time this was though.

To repeat what another person has said, the Canadian side is better than the US side (Sorry!), and **definitely** take the Maid of the Mist tour. It's magical sailing right up to the wash of the falls.

Depending what you like, the town itself is a bit like Las Vegas. I've never been to LV, so can't compare, but it did strike me a lot like Blackpool. Very tacky, loud etc. Not entirely befitting of the natural wonder you're after, but that's my opinion.

But, 30 mins outside of Niagara Falls is a town called Niagara on the Lake. **It's beautiful!!!!** The buildings are brilliant, the pace of life is so quiet. We were told to go there by a resident of Toronto after I got caught on camera as they filmed some sort of candid camera show. 100% would recommend you go there, even for an hour. You won't be sorry.