Palm Beach is great. Busier than many beaches, but we did not find it TOO busy and we hate crowds. Lots of nice water sports to choose from, and easy to get to by Aruba bus (cheap and quick - our family rode for under $10 total one way).
Grand Turk is a little touristy for us, so we decided to skip Margaritaville where it was too packed with throngs of people. We headed down 1/2 mile to Governor Beach. We played with Topher the Dog, rented some snorkel gear and paid for our kids to jump on the water trampolines. It was a very inexpensive, relaxing and fun day.
I can say enough about the Saona Island excursion through Pro Excursions. It was flawless and unforgettable. We took a speedboat through some mangroves, snorkeled, visited the private beach on Saona with a Dominican lunch included, and then stopped at a natural pool covered in starfish on the way back. Best family experience we have ever had.