Very beautiful park and beaches! You can 1 of 3 ferry trips, $7.50 round trip, to three different spots on the island. You also don't have to return on the same ferry you left on, which makes exploring the very fun.
You can walk around the trials, see nature and great views on the skyline. There's a amusement park, but its definitely catered towards younger kids and its anything exciting for adults or older kids.
You can rent bikes or boats and there's also a sizable shore line you are lay out in the sand on.
My advise, grab some food for a picnic from St. Lawerence Market and enjoy the serenity and the view.