You are right the Santa Dominigo tour is an interesting history lesson about Columbus and his son. We took that excursion when we stayed at Boca Chica.
The heads up about Double the Fun tour makes me want to shift towards that. However, it depends on how time my wife wants to shop.
So far our itinerary looks like:
Grand Turks - realx at Jack's Shack and enjoy the food and rum shots
La Romana - Altos de Chavon view its culture and explore the local crafts
Aruba - Open air bus tour
Curcacau - Double the Fun tour (assumption of limited shopping)