I was here too early in the year on a weekday for many of the amenities to be open but had a lovely time wandering the island and enjoying the lake. I even got to see Canadian geese in their natural habitat!
The island is HUGE so renting bikes or jitneys (or y'know bringing your own) is a great idea. The amusement park looked fun but it was closed. Most of the food sellers were closed or looked...very child oriented. When I did try to buy a bottle of water from a vending machine, it wouldn't take my coins. So I suggest bringing your own picnic with you.
I loved watching the spaces getting spruced up for the summer season and really loved wandering in Franklin Children's garden. And loved watching the little kids reaction to a wide open green space...Time to start running!!!