I'm not suggesting that an unreasonable amount of the green space be converted but there is some that can be.
Waterfront Toronto's upgrades should include centre island. The park space is nice but its not world class. When you first leave the centre island ferry (usually with a hoard of people), the first thing that you're greeted with is a cheap concrete structure housing a pizza pizza, a concrete slab with an antique cannon atop of it, and a bunch of prefab structures that are meant to be tourist info centers. It's extremely obvious that no care was taken into its presentation as the chief gateway to the island.
For those who say leave it as be, I remind them of the following things, first development has already been allowed for both the Toronto Marina and the Royal Canadian Yacht Club (nice exclusive facilities for those who have a boat) and second with centreville amusement which owns concession spots around the island, by the beach and Hanlans point, cheap run down ones.
Also remember that centre island is chiefly used during the summer with impressive numbers of tourists and residents alike, plays host to events like the dragon boat races and marathons. Right now the economic potential for this is being overlooked. This could be a major source of revenue for the city, this could help pay for better community centers, schools, urban improvements, infrastructure updates. I find it unusual for people to recommend it be left as is so that they may return to the island once a year and see it "unchanged".
With grand landscaping (think millennium park, central park, the grounds at Versailles), maintenance of all major wildlife destinations and green spaces, the addition of waterfront commercial spaces, Toronto island could truly become world renowned.