Keep in mind, there is some good snorkeling in Curacao too. Once you are onboard if you still don't know what exactly to do, watch your TV and go through the shore excursions. Also, beforehand, use the cruiseline's web-site and look at them all, lots of interesting things to see and do.
In Aruba... DePalma island is just off shore, and owned by the Renaissance hotel, which runs ferries back and forth all day. You used to be able to buy a day pass at the hotel in downtown... not sure if that is still the case. But, if so, it makes for a great beach day.
We've also done an ATV tour on Aruba that we really enjoyed.
Curacao - wandering around town is simple and pleasant, or you can grab a taxi to Seaquarium beach.
I did ship tours from Carnival on both. Did Jolly Pirates snorkel trip in Aruba, which is a combo party booze cruise with 2 snorkel stops and a swim stop. Water was too deep and choppy for me to snorkel I'm experienced and it scared me but we had a blast dancing the limbo line, If I had it to do over again I would have just done a land tour or gone to Eagle beach. In Curacao Carnival had a 7 hour tour that took us to Boka Tabla where the waves crash, 2 pristine beaches for photos, 2 museums, and the Curacao Liquor factory. It was one of the best ship tours I've ever done! Amazing!:o Curacao is my new favorite port so much to see and do there! I hope to return!
Carnival Mardi Gras 1987 (Bahamas)
Carnival Legend 2005 (Exotic Western Caribbean)
Costa Mediterrenia 2005 (Eastern Caribbean)
Carnival Pride October 2006 (Mexican Rivera)
Carnival Legend 2007 (Western Caribbean)
Carnival Glory 2009 Western Caribbean
Carnival Liberty 2011 Eastern Caribbean
Norwegian Spirit 2011 Western Caribbean
Carnival Miracle 2012 Southern Caribbean
Carnival Breeze 2013 Exotic Eastern Caribbean
Carnival Valor 2014 Southern Caribbean