Congratulations!!! Number one question - are you and your fiancé(e) divers or interested in diving? What about snorkeling?
I can only speak to Grand Cayman, and it was 2004 when I was there for a week-long dive trip, but 7 Mile Beach is beautiful and the diving is world class all around. Stingray City is a famous dive/snorkel that you can do there. IIRC, it's pretty toasty there in the summer, but certainly nothing unbearable. A rental car is very helpful for getting around and a must if you are staying in more remote areas. Definitely good food to be had as well!
I have heard a few people express concerns about crime in Aruba - a friend was just there and had his new PDA stolen - but have not been there myself.
Happy to give you more info on GC diving if you are interested!