I posted a response for you - but it seems to have disappeared! I agree, mid-June is a good time to visit - not quite so hot, usually.
I live so close to the Falls I can see the mist from my back yard...we do LOTS of tourist-trips with family & friends as they visit. Be aware, if you have a rental car that parking at the Falls is controlled and expensive.
I agree also that you must do the Hornblower trip, and the Butterfly Centre, and visit Niagara-on-the-Lake. NOTL has lots of boutique shops, good restaurants, many wineries, and even a horse-drawn carriage ride!
Flying into Buffalo gets you closer to the Falls, and a drive along the Niagara Parkway is fun. Flying into Toronto means you are about 2 hours from the Falls...but if you want to see some of Toronto as well, that is a good option.
Do call if you get to the area and if I am at home, maybe we can meet - even if only for lunch!
Grammie to 10 darlings, ages 5 to 25 years plus 1 step-darling (she is 15)! Mom to 5, ages 47 to 54!