Pallabazzer recommends seeing the historic center of Florence at different times of the day. In the early morning, you'll get to experience it "without noise and the pressing pace of crowds." Midday brings droves of visitors, but the destination is "bathed in sunshine." In the evening, "the lights of the street lamps stretch out over the Lungarni [the streets along the Arno River], creating a truly magical effect."
My husband and I visited Niagara Falls on both the US and Canadian sides. My favorite experience was on the American side, but there was more to do on the Canadian side. For the best trip, I recommend spending time on both sides of the Falls. When researching a trip to Niagara Falls, you'll notice there's a fairly heated debate about whether the Canadian or American side is better.