Head out to Mount Nemo Conservation Area for hiking, rock climbing, and a spectacular view of Burlington and the GTA. I recommend visiting after breakfast when the crowd hasn't quite picked up yet, it's a popular area amongst locals!
I've been to Niagara Falls more times than I can count, and it is ALWAYS simply incredible. I've stood by the falls for hours, until soaked by the spray. I've also been there when the smaller section (The American falls) was frozen.