All three cities have merit, however Toronto would be the biggest bang for the Time frame . Inner city travel on the transit above and below ground is excellent. Tours of Niagara region are available as well as toronto city tours.
If the weather is good try to find an evening boat tour of the waterfront. The lights of the city are fantastic. The Canadian National Exhibition runs the last two weeks of August. Each evening about 9.30- 10.00 p.m. there is a fireworks display on the Lake Ontario waterfront of the C.N.E.. It is best viewed from a boat. Have fun and enjoy !