Niagara falls

seen from the Canadian side :-)
Also take a look at USA side

There are day trip tour options from Toronto but I recommend a weekend staying at Niagara Falls for two reasons.
First: it is a laid-back way of visiting it so you can take a train from Toronto and 2 hours after here you are :-)

with enough time to taste some kind of beers :-)

Also I recommend double-deck bus

because beyond turning around some queues you can see the falls behind them :-)

Cross a lake at a cable car

get the camera wet

Second and most important: you can taste Niagara wine while looking at the falls :-)