My family went on the natural pool, cave, baby beach jeep tour. Ernie was our tour guide. Ernie was funny, knowledgeable, and personable. He made the experience top notch.
The whole experience is a tremendous value. You get to see three sights with lunch, gear and a guide. It would not be possible to recreate the excursion on your own for anywhere near the price. Even if you could, you would still not get all the information that the tour guide has about the attractions and about Aruba in general.
My wife originally wanted us to rent a jeep and go to these places ourselves. I do NOT recommend that. I have some limited off-road driving experience and I would have had a tough time making it up the trail we took to the natural pool. To make the drive yourself without taking all day or potentially rolling over you would need to be an expert at off-road driving in rocky terrain like Ernie obviously was.
Visiting the cave would not have been nearly as interesting without a guide. Ernie pointed out a lot of things that would not have been obvious to the average visitor. He had a lot of knowledge and was happy to share it.
Baby beach is a pretty far out from the tourist areas. It would not be cost effective to try to get a taxi out there. You could rent a car, but then you would not be able to get to the natural pool, you would need your own snorkel gear, and you would need to buy your own lunch.
Truly the best and most cost effective way to get to see these sights is to let Ernie be your transportation and your guide.