Beth, If you are interested in doing the southern Caribbean, there sre some wonderful beaches to be had...On a 7 day, you would need to leave from San Juan...Grenada has Grand Anse Beach, considered ot be one of the most beautiful....Thiis itineray iis port intensive but, worth doing...other nice beaches too in this area.
The western is more geared for water enthusiasts, so I would say, for you guys, maybe the eastern will give you the most bang for your buck...
St Thoma has Megans Bay, and if you ferry over to St. John, you will find postcard beaches....Antigua, has 365 beaches, and, surely one would be perfect

Half Moon Cay, Carnival/HAL's priveate island is a slice of heaven....
Now all you have to do is decide

If the time period of after Thanksgiving to mid Dec could work for you, there usually are some amazing deals out there. you can cruise, while othere Xmas shop