Peter Island ha a beautiful pool and beach. But, I would pick Anegada.
Here are some thoughts-
Rent a car and tour the island. There is a iguana preserve in the settlement. You can hike in to see a flock of flamingos. Take them bone fishing. If they don't snorkel yet, Cow Wreck is a great place to learn.
Just a few ideas. I also love the treasure hunt idea..


Mike
"The journey is the thing." Homer