Take at least two days to see the sights then you won't be rushed. There are a number of attractions that have package deals along with transportation. See:
InfoSeeing the falls by day and by night, the shops and museums on Clifton Hill above the falls, Maid of the Mist, Butterfly house, the Whirlpool, etc., etc.
The Welland Canal, as mentioned above, is very interesting to visit.
Be sure to have your passports or passport cards with you.