Well, Antigua claims to have 356 beaches... so you can do a different beach each day of the year... but yes, they have great beaches there. But the beach at shipwreck cove on Barbados was really nice too. The sand there was white and so fine it was like flour.
If you liked snuba, why not try the really thing and do a beginners scuba dive trip. They have those available at a couple of places you mentioned. You only go into 25 feet of water but to be non-tethered is a great feeling.
I can't help you with the zip lines as we have not done any.. my youngest is not comfortable with heights and roller coaster rides and that kind of combines both of those elements.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007