I've only been to Aruba and Bonaire, so I'll offer my suggestions for them.
Aruba:
Submarine ride or semi-submarine ride to the wreck of the Antilla:
http://res.atlantisadventures.com/tour_i…
Butterfly Farm:
http://www.aruba.com/whattodo/butterflyf…
Bird Sanctuary:
http://www.aruba.com/whattodo/birdsanctu…
If you go to the high-rise resort area on the northwest shore of the island, you'll find a wealth of water sports, including banana boat rides, parasailing, snorkeling, scuba, snuba (like a cross between snorkeling and scuba -- no certification required), windsurfing, kite surfing, etc.
You can also shop for souvenirs in downtown Oranjestad.
Bonaire:
Donkey Sanctuary:
http://www.donkeysanctuary.org/index.htm…
Scuba diving or snorkeling (better than Aruba)
A scenic drive up the northwest coast offers beautiful ocean views.
Lac Bay: a calm and shallow bay on the east coast, excellent for swimming and windsurfing.
The Sorobon Beach Resort -- the island's only nudist resort
Landsailing:
http://www.landsailingbonaire.com/
You won't have time for all of these activities, but I'm trying to give you all of the best options to choose from. My girlfriend and I spent three days on each island and didn't do all of those things. On Aruba we did the semi-submarine ride, parasailing, swimming, snorkeling, and souvenir shopping. On Bonaire we went kayaking and snorkeling at the Sorobon Beach Resort, went to the Donkey Sanctuary, and took the scenic drive up the coast. I enjoyed all of those activities and would like to try some of the others, like landsailing, scuba diving, and the butterfly farm.