Here are some ideas:
-Ribfest
-TD Toronto Jazz Festival
-CHIN Picnic @ CNE
-Toronto Island Music & Arts Festival
-Pride Parade
-Midsummer Night's Dream @ High Park
-Cavalia's Odysseo
Based on experience from previous years, I'd have to say the highlight would be the fireworks at night. And celebrations and fireworks are happening in many towns outside of Toronto as well. The best place to view fireworks in my opinion is where you can see the fireworks near a body of water (i.e. the lake) so there are no obstruction and you can see the reflection.
Hope you will all enjoy the long weekend, I know I will :)