Beach toys! Sand toys are great if you can fit them or pick them up at a Wal-Mart or something while you are there (Cancun has a Wal-Mart, i notice you are going to Aruba, and i have no clue if there is one there) but also consider deflated beach balls, swimming rings, and rafts...they take up hardly any room when deflated and are so fun in the ocean!
You can also consider things like coloring books, flip flops, or even little squirt guns.