Not sure where you are staying in Aruba, but I actually got my open water cert in Aruba from Aruba Watersports. Seeing as there is a dive shop between every hotel on the beach and then some, it can be a bit difficult to choose from.
Now you’ll have to take my experience for what it was, but I thought they were pretty good. Looking back, I still think that and would continue my training with them. Unfortunately I don’t live there. The boat was nice and you really aren’t more than 15 minutes from the dive sites from the shop.
As far as dive sites, the Perdenales WWII wreck is probably one of the most popular ones there. I think I remember this one the most because I had a close encounter with a giant moray there. The rest I’d have to look up in my log book.
Make sure that you bring EVERYTHING you might need. Supplies on the island are expensive. And bring a healthy food budget if you aren’t at an all inclusive place.