We've been to Treasure Island a few times.
For kids its absolutely fantastic.
Its got heaps to do without the "tourist trap" markets, marketing and spruiking that happens on the mainland.
The island isn't paved with travertine and as glamorous as some of the mainland resorts, but the friendly staff and perfect views from every room make it a special place.
We won't ever go anywhere else now.
If you do go to an island, pay the extra and take the chopper, it saves heaps of time and is an event in itself. (Try to get Jenny to fly you (island hoppers) - she's very easy on the eye!!!!)
Whatever you choose, you will love. its a great place with great people.
Cheers
Brian