I always wear a light sweater on flights because I'm usually chilly on planes, and when I go to Aruba, I dress light for the gorgeous weather there rather than for the weather that I'm leaving. I'm not sure that I remember ever needing one anywhere on the island, but I'd rather have one available than not.
Eagle Beach, exploring the "Outback", Madi's tour to the Natural Pool , dinner at Papiamento's
I layer a long sleeved white shirt over a tee on the plane, along with a windbreaker jacket for warmth leaving New England. I've used the white shirt for sun protection, never needed the jacket! I also pack a couple of pashmina shawls to use at night, but have only needed them for indoor restaurants, and we usually eat outside -- a nice tank top is fine fot that, as it really is HOT! I don't think I'd bother taking a sweater or sweatshirt.
Baby Lagoon, T Docks, BA Beach, Palm Beach, Hooieburg gold mines, diving the reefs,
I grew up on the Island and assure you that you will not get cold. The trade winds keep the Island cool. While growing up there I never wore anything but a T shirt and shorts unless I was dressed for church and those were to many clothes on for me. If your going to do the Island thing, T shirt, shorts, straw hat and flip flops. If your going to do the tourist thing, dress as you would at home in summer.
I always take a light sweater with me to Aruba. While I don't use it often, there have been a few evenings where it was nice to throw over my shoulders sitting outside, and as others have mentioned, to use inside restaurants and casinos where the a/c is on. Otherwise, have not had a need.