We went about 5? plus years ago and stayed at the KOA on Lundy's Lane in Canada. We were all towing 20ish foot trailers (3 families), and found the campsites to be just fine.
As mentioned, the Niagara Falls area is VERY touristy - it is kinda depressing when you first arrive expecting the pristine waterfall in the wilderness that you had been seeing in all the pictures. Just go and enjoy it.
We greatly enjoyed the Skylon Tower restaurant, which is practically right over the falls. Spectacular view, and for 3 families the buffet was great and we really put a dent in their profits that day.
Something that myself and 2 of the boys enjoyed was a tour of the power plant a few miles up the river. You take an elevator something like 27 floors down to see the generating room and display room showing how everything works... they don't actually get power directly from the falls anymore, there are canals and reservoirs that are used to create hydro power. A lot of the old abandoned generator buildings are still near the falls.
At the visitor center on the Canadian side, you can get up to within about 10 feet or so of the top of the falls. It is pretty cool to just relax and watch for awhile!
The Maid of the Mist trips - I take it they are now called Hornblower Cruises? - is worth it. The boats go right into the mist at the base of the falls.
When crossing the Rainbow Bridge at Niagara Falls, be ready with everything you need for crossing the border, and be ready for traffic. Worst by far was the return trip - traffic was backed up horribly - almost 3 hour wait just sitting there. Nothing you can do about it, except maybe try to time your crossing different than we did!