I just got back from both islands. This time there, in Curacao, we went to the Sea Aquarium. It's a 15 dollar cab ride, and 15 per person to get in. You can easily spend all day there. They have dolphin shows, and sea-lion shows. They have 15 nurse sharks in a fairly shallow pool. If you get there early enough you can see them feed. At the gate, you get a map, and a schdule. They also offer extra fee activities, like dolphin encounters, shark and sea turtle scubas, etc. I would also recommend that you explore the shops in Willemstad. Some good deals to be found.
Aruba.....take an off road vehicle adventure of some sort. I prefer the ATV one, (i used http://www.ranchonotorious.com/quadracer.shtml
) . These take you to the windward side of the island, that has NO commercialzation at all, and is an amazing desert landscape(it has no buildings at all for miles at a stretch).
Curacao and Aruba are my 2 favorite islands. David Landry is correct about the Commercialisation of aruba. Much more hotels, high rises, casinos, and the like.
Curacao has beautiful dutch influenced towns. Brightly colored houses, and the magnificant Queen Juliana Bridge. Curacao is more industrual than aruba, though. It's cool at night to see the giant flames coming off of the smokestacks casting orange flickers skyward.
David Landry.....Just Curious about your advice... the only rule of traveling that is more important than "don't drink the water" is "don't drink dairy products". Is this really a good idea? Un refrigerated milk in a foreign island nation? eek! Is it really ok?