Sounds like you might be all set without a car. It really depends on what you plan to do, but as you indicated in your post and what you are planning, you probably don't need one.
Some Caribbean islands we stay more at the resort and don't need a car rental but in Aruba, we feel different. We have always rented a car (4 times) when we have gone. We prefer not to wait on others schedules to do something, like the bus. We also like to be able to get groceries without lugging them on a bus. The amount we go out and about for a taxi (for 4 people) would add up to what it costs us for a rental anyway, so we prefer just to rent, it's easy enough. We've always just used Budget, it's right there at the airport and I have always been able to google discount codes that have worked to find a reasonable rate and made the reservation right online. We also enjoy exploring the island at our own leisure, we have gone to all the tourist spots that they have excursions for on our own. We have great memories of our "adventures". The island is small and safe enough we have never had any issues or problems. We love Aruba and the Marriott's there are both great, have a wonderful trip!