Everyone has given some great ideas.
My advice is book your site early no matter where you stay!
July 1 is the Canadian holiday and July 4 is the US holiday. Canadians will camp in US because all Ontario parks are full for the long weekend and folks from the US will camp in Ontario for the 4th because all US parks are full.
we were in a Michigan state park last year for our May long weekend and 75%+ of the rigs had Ontario plates. The folks from Michigan I talked to said they were heading to Ontario for the next weekend to avoid the crowds.
Enjoy, have been to Niagara Falls many times over the years and we always discover something new every time.