I think Rayon is a very good option would also throw in Crystal Waters...they offer villa type accommodations and will cook meals for you guys. One house could be the "main" house for gatherings and such. It's next to Margaritaville which maybe a plus for the younger people in the group. I would suggest each family book their own airfare. You track the cost and when airfare drops send an email/text.

Regarding excursions, send a list of what's available with a brief description and allow family members to vote. Then combine the tours with the most votes into say two day trips.