Glad to see you are giving Aruba a visit...you will love it !! I will try to answer the questions I can and I am sure everyone else will pitch in and give you their versions.........many different ideas from everyone, you won't know where to start first....
I always rent a 4x4. My husband and I first visited on a cruise and rented one and took the chance and toured ourselves, we had a blast. It has become a tradition to go around the island every year to see if anything is new, so my suggestion is yes. But if you are ont too adventurous, rent one for 3 days and see the island and then just use a cab to get around to downtown or dinner.....You can even use the bus which is cheap, we have and have never had a problem.
If you pay in dollars you will get dollars back, however you may get change in florins. You can use either....you can get florins when you get there or not, depending on your preference. I never exchange for florins.....just keep bringing it home with me...up to about 40 dollars american and might take it with me in november and cash in !!!
Not sure how late the cabs run, but yes they are okay...I have seen folks talk about a cabbie named "Bully" and how great he is...a little help please.....
You can get a bus schedule at the hotel also, they run all day, but again since I use a jeep I don't know their schedules..........I do know I have paid the cabs in us dollars though as well as the buses.
I love El Gaucho, but you will hear varying opinions on that one. I have never had a bad meal and the meat has always been tender and cooked perfectly. A matter of opinion I guess. I would day a must, but I would also suggest Madame Janette's......YUM !!
Shopping is great in town. When I was there last year the shops up towns by the high rises were just opened..........nice but very expensive !!! I don't go to shop and don't bring home gifts so I really don't do much shopping anymore. Good jewelry shops downtown with really good deals though.
Some restaurants automatically but a gratuity onto the bill which I don't care for because they say it goes to the entire staff and not just the waiter. If I see that I slip the waiter 10or15% more so I am sure they get what they deserve, but just what I do. I would say to tip as you would anywhere else.
A must see..........Natural Pool, but hard to get to....quads are the best and safest so if you're into it ....go for it. I would say rent a 4x4, get a map, start very early and start at the hotel, head for the light house, alto vista chapel, goldsmith ruins, then natural bridge........then start from the other end the next day...baby beach and snorkeling, quadrikri caves, love tunnels, ....OH heck, just go for it !!!! Take a sft cooler with you from here in the states to use and fill it with lots of water and spend a day or two exploring......you will come back every year.....I promise !!!!