I would suggest renting a car for a couple of days if you've never explored Aruba.We use the car to stock up at the grocery store, but thats something you also can do by taxi. Spending a day at Baby Beach is a must and it's at the other end of the island so the car was great for that.. We drove around the island and stopped at several beaches to snorkel. You can get a map which shows all the sights to see which you can do easily in a day. We also used the rental car to dine at The Flying Fishbone which would be quite an expense if you go there by taxi. Getting a rental for a few days is probably your best bet. We have gotten a rental car for the week and there were days it sat in the parking lot untouched. This year we plan on getting a car for 2 or 3 days.
I suggest renting a car for some of the days you are there. I tried to plan for the days we didn't need a car and just rented for about half the visit or so. It's definitely a drive from the Marriott properties to anywhere else on the island and we had a lot of fun taking various drives. We rented a Jeep so we could go more places, and only had serious off roading when we looked at the "road" to the hidden pool. Note your rental agreement carefully, a lot of the sand is off limits and there are other catches. We got a rate about half of what the hotel was asking in the lobby by just looking around and doing our own searching online.
I just got back from a week at the Surf Club and in the end I'm undecided whether I made the right choice to rent a car for the entire stay.
On one hand, we had the ability to have dinner downtown each night but guess we could have taken the bus into town, as inconvenient and time consuming as that may have been.
On the other hand, there were days when the car just sat in the parking lot.
Add to the mix that getting from the airport to the hotel costs about $25.
Baby Beach, which I think is just OK due to it being right next door to a oil refinery, is 22 miles from the hotel and would be expensive by taxi.
I rented from Thrifty at the airport for $55 a day.
The location of the Marriott and Surf Club are good; 90% of your destinations are on foot (via the beach), by bicycle (you can rent MTN bikes to cruise around on at the Marriott beach), or by short taxi ride (down town and some restaurants on the island's interior).
I might suggest renting a car for one day. Baby Beach is great, and yes, there is an Valero oil refinery there, but the view there is UNDER water. Then spend the second half of the day at Hidden Pool... 4x4 is recommended other wise it's perhaps a 4km walk from the improved (dirt) road and parking lot. The crabs are fun to watch there.
My 2 cents,
You are staying at a timeshare, as I have when going there, and having a rental car at your disposal is always good. If you need snacks/food/booze/drinks/water for the week, you have to buy it. The trip to the supermarket will cost you cab fare. The trip to reataurants if you want to eat on other parts of the island other then the High-rise district (where you are staying) will cost you cab fare. A tour of the island will cost you cab fare.....and on and on. I always rent a car IN ADVANCE before I go to the island. It's much cheaper that way. Don't buy into the 'get insurance to cover the possibilty of accidents, etc' baloney. If you are a good driver, you don't need the insurance. Even if the car sits for a good bit of the time, it is always good to know that if there is a sudden desire to leave the pool/beach area to go someplace else to have a lunch/dinner on a waterfront location other than where you are, you can. I always rent a car.
No matter what you decide, enjoy the stay...it's a beautiful place. We have many friends on the island and always enjoy our visits there.