It's a romantic start to an evening because you can see the city skyline sparkling with lights. During the day, you can roller blade, walk, or bike the kms of pathways that take you around the island--even over to the south shore beaches.
As I recall the restaurants are kind of grim fast food fare, so bring a picnic and a towel or blanket (lots of geese in the grassier areas).
There's a petting zoo and small amusement park called Centreville. The log ride is ALWAYS fun.
You can buy ferry tickets online now--probably a great idea as line-ups can be long. This website has some additional advice on how to get there by transit as well as extra sights to see. Take the Centre Island or Ward's Island ferry. Nude beaches (for men) at Hanlan's Point. A reminder to check when the last ferry leaves the island so that you don't miss your ride home!
Of course, you can always kayak over to the island... and there are some fun canals to explore once there. Rental kayaks at Harbourfront Canoe and Kayak rental located S.W. corner of Queens Quay W. & Rees St/Robertson Crescent neighbours to the Marine Police Station.