I love Toronto Island because my sister got engaged there (how cute is that? Shout outs to Danielle M.)
Also, I love Toronto Island for bike riding. You can bring your own bike across on the ferry or rent one there (but it will cost you $10 an hour. Don't get the tandem bikes - they're much more expensive, and hard to ride).
Food options on the island include Pizza Pizza, pop machines and the dicky dee man... I suggest stopping by St. Lawrence Market first and bringing a picnic.
Try and go early in the morning to avoid weekend lineups. Don't go on a long weekend, you'll be squished into a super sweaty lineup trying to get on/off the ferry. Also the City put in two new automated ticket machines that people have been praising - they suck. They're super confusing to work and people take about 15 minutes to figure out how to buy a ticket. Skip that line and just wait in the big clump of people in the middle.