@amberwc
First off, a great source of information are the Past Navigators that are on Scott's forum
Personal Navigators, not sure if you have checked them out for your previous cruises, but they give you an idea of what activities are available on similar itineraries.
My wife and I are also not beach people so we usually don't do excursions, we spend most of our time just wandering the ports shopping and visiting friends we have made over the years (we have done 12 western Caribbean cruises). I am sure other members here on the boards can make suggestions on excursions that you may enjoy.
For the kids there is always the kids club which it sounds like they are a great age to enjoy. The Fantasy and Dream have the Mid Ship Detective game that we see lots of kids enjoying on our cruises, but it is for all ages and the entire family can participate.
I am not sure which western ports your cruise will have but if you choose to do things on your own I can share the DW and I are very comfortable walking around on our own in Cozumel and Grand Cayman in the main shopping areas, in Falmouth the port is also very nice to walk around, but we do not feel comfortable leaving the security of the port. Jamaica is the one place we did take a Disney excursion - we did the Green Grato caves. This would be good for the kids, and if you are OK walking uphill/downhill and on uneven surfaces inside caves it was very interesting. I am not sure how your Dad and his friend would make out.
If you are craft beer drinkers there is a small microbrewery, Cayman Island Brewery in Grand Cayman that you grab a cab and visit, they do a tour and have brand new tap room that they completed just after our last visit. It cost us about $25 each way plus tip and they are about 20-30 minutes from the port. We communicated with them via their Facebook page. They will call you a taxi to return to the ship, you just have to be mindful of time to ensure you get back in plenty of time to the ship on time.
If your Dad does get married before the trip, be sure to call the cruise line and let them know so they can make a notation on his reservation.
Have you had the opportunity to eat at Palo on your past trips? If not I highly recommend the brunch. We have never eaten dinner their because we enjoy the brunch so much. We plan on trying dinner on our next cruise.