If you will drink any alcohol...for the love of god get it at the airport before you walk out in Aruba. There will be a line, but if you try to get a bottle of titos anywhere else on the island it will set you back $80. The airport has US prices. Also if you have a Rumblr you usually drink out of I would bring it, the hotels will fill your drink up and will be able to save money that way.
Its very expensive there compared to other islands I have been to. Even more expensive than Hawaii.
Sunscreen and plenty of it.
We really enjoyed renting the ATV and going to the very east side of the island and going nuts.
Did I mention how expensive everything is there?
If you can bring your own Umbrella/Shade. If you visit other beaches they will charge $40-100 for shade.
Bring goggles, the water is crystal clear. The beaches near the lighthouse on the west side have strong waves. The east side is very calm.