More bike and hike trails would be my choice, perhaps with some interpretive nature guideposts, etc. I found myself wanting to explore more of the island than the existing trails would allow.
If you want more and different experiences, horseback riding on the beach, segway tours (preferably off-road), a bigger playground/spray park for kids are all possibilities. If they want to go for something really different and new for a private island, expand the stays into the night and have a campfire-type show (Mickey's Backyard BBQ-esque) on a new section of the beach.