HTO Park is great!
I particularly like the urban beach--what I come from. It's right near the lake, across from the Rogers Centre/Skydome. It's got these great yellow umbrellas and white Muskoka chairs so you can sit and also have shade. Bring a book and just park yourself for the day.
There's usually skateboarders or bikers/runners in the area, and people walking along the water, so it's great for people watching. If the beach/sand area is packed, you can hang out on the grassy little hills.
Tip: Come before 12, before it gets busy, to grab chairs (which are chained to the umbrellas). Even if you can't grab a chair, just throw down a blanket on the sand and enjoy the lake views. There's also a water fountain nearby if you get thirsty and want to fill up.