Beaches are amazing. Walk the dog, rollerblade, bike, play some vball (assuming the gang that is Warren & Co. have not taken over every single court :P ), sit on the rocks, make out in the bushes - don't step on needles - it's all good fun!
They say it's safe to swim in lake Ontario now? Give it a try. let me know if you develop a funny rash.
If you're more of pool kinda person, try the Olympic pool. I went a couple times and loved it - they have a kiddie area, diving and a massive pool (Olympic size, like you probably guessed). In the summer it gets packed. Wear slippers in the showers and on deck, or warts.
HAVE FUN!!! Enjoy your city, Toronto is amazing so make the best of it. I love coming here to play with the doggies.