Niagara Falls

Impossible to give a reasonable recommendation without knowing more details. Especially: how long is your trip, and what all are you trying to include in that time?

No matter how long your trip is, you would be skipping other things to do the Niagara Falls daytrip. If your trip is short (3-4 days), that could mean missing out on other worthwhile things. But if your trip is 3-4 months long, then why not? All depends on how you balance the value of visiting various places, how much time you have, how much area you are trying to cover, and (at least as important) your own preferences/priorities.

Go to the falls and take the boat out into the spray below Horseshoe Falls!!!

Niagra Falls is definitely worth a couple of days. Also, they have some good wineries in that area.

You can pretty much see what there is to see in a few hours, so if you're already staying in the vicinity - am thinking in particular of Niagara-on-the-Lake - then a day trip is quite doable.
As with every popular tourist attraction I'd suggest getting there as early as you can to beat the crowds.

Very doable on a day trip from Toronto if you have a car.

I have only been to the US side of the falls, and that side is worth half a day at most. Maybe the Canadian side has more attractions and nicer landscape, but the US side had your typical cheesy casino etc (at least when I went, which was over a decade ago). Google what there is to do on the Canadian side and see if it's worth it to you to give up a day or more from Toronto.

Depending on how much time you have, I would recommend at least a short stay in Niagara-on-the-Lake and a day trip to Niagara Falls from there.

Your hotel in Toronto will have information on bus tours. Some itineraries are more wide-ranging than others. Public transit is also possible. To get beyond the Falls requires some sort of private or tourist vehicle. The Falls surroundings include a manicured park but also honky-tonk tourist draws, including a big casino. The drive down river past the Whirlpool rapids to Niagara-on-the-Lake is scenic. Along the way you can see historical sites relating to the War of 1812-14 (versus the US) which we, and only we, think are important.

I have done the Maid of the Mist boat ride and the thing where you walk under the falls on the Canadian side wearing raincoats twice, and I would do it again. Definitely go to the Canadian falls and have fun with it.