back when i was a kid we lived in southern ontario.
some things that were always super fun to do were...
royal ontario museum & planetarium (i absolutely LOVED going there)
canada's wonderland
jays games
the cne (but that is later in august)
the toronto zoo
if i was to go back now i'd definitely want to visit the hockey hall of fame
***edit - something you might want to look into - getting a "city pass". when my wife and i went to seattle we got one and it was a great way to save a bunch of $ and see lots of attractions.
https://www.citypass.com/toronto