Definitely rent a car FOR ONE DAY to tour and see the entire island. Make sure it has A.C. Do not attempt to walk too far away from the beach. The island is very hot once 1/2 mile from the water. Enjoy the casino. Even the slots pay out fairly well.
We have been to Aruba several times and we found taking a full day tour gives you a chance to see the Whole Island. Use the public transit for any downtown trips, its cheap, clean and the people are friendly. Do go snorkling! Do take an evening on the the party bus great time.