I was born and raised in Western New York, so I know the best and worst of Niagara Falls. You don't have a lot of time, so here are the best sights at Niagara. Cave of the Winds (if Open) Bridal Veil Falls and Terrapin Point on Goat Island between the main falls, Maid of the Mist from the American side, any of the towers on the Canadian side, The river Road in Canada, The Aerocar at Whirlpool Rapids, Niagara Glen Park just past Whirlpool in Canada, and either the Adam Beck Power Plant or the Robert Moses Power Plant. Take the Rainbow Bridge into Canada and the Lewiston Bridge coming back to the States.
There are a lot of tourist traps on both sides of Niagara Falls, but the sights I suggested are all either inexpensive or free and will provide good views and pictures. If you like hiking, Niagara Glen will give you some exercise, too.