I love Harbourfront in the summer. Love, love, love. They have great free events in the summer -- tons of concerts with actual big name bands and performances.
It's a gorgeous place to take a walk, but is obviously much cooler than the rest of the city since it's close to the lake. If you're dying of heat in the summer, head over to the Harbourfront for a walk and you'll cool down in no time. Plus they've got some great parks for kids to play and it's just an all-around fun place to be.
Note for bikers: They've got some great bike paths around the area so take advantage of them and explore!