Castaway Cay was great! Disney knows how to do islands. We did one excursion - the bike rental. It was definitely worth it. The bike patyhs are beautiful, and there is an observation tower you can climb which shows you a fantastic view of the island and the ship.
The beach was crowded, but the water was beautiful. The water was cooler than we expected, but not enough to keep us out! The food at Cookies Too was excellent, and the ice cream on the island featured a few more flavors than on the ship.
The character greetings at the post office were great! We didn't meet Captain Hook but we did meet Chip and Dale, which made our son happy.
The post office was closed when we tried to mail off our postcards, but Guest Services on the ship were happy to mail them for us.