I agree with the previous comment that you should do your research in advance of getting to Aruba as it will leave you with more time to relax while on vacation. That being said....
I would book your excursions in advance. Indeed, some of the companies offer discounts if you book in advance/book online and you will be assured that your chosed date will be available if you book in advance.
In my opinion, the full jeep tour is well worth it as it is a great way to see Aruba. However, I would recommend that you go with a company that travels to the Natural Pool as part of an island tour. That being said, I thought the ATV tour was more fun than the jeep tour. Kini Kini tours offers a great full day ATV tour that you may want to investigate further.
For the sunset sail, you may want to try Pelican Adventures. I believe it is $42 per person (i.e. reasonable) on a catamaran (i.e. nice boat) and it is not too crowded (at least it was not when I went).
As for renting a car, if you are planning on doing the full day jeep tour, you will most likely be going through the National Park and visiting the caves on the tour. Hence, you may want to rethink whether it is necessary to rent a car when you are there.