Cas, welcome to Cruise Chat ! I'll bet you are getting anxious for your cruise; thats a very diverse age group of cruisers..Not knowing what you all would be interested in it is hard to say..also, depending on how much your individual group wants to shell out for outside tours.
We were in Grand Caymen in February and did a stingray and snorkle tour with an outside group on a catamarand for $45. each. Met them at the pier and they took us to the marina where we boarded and sailed out to swim with the stingrays in approx 4 feet of water. It was a little windy and waters a little rough but there were plenty rays to feel and interact with. Then we did a snorkle on a reef nearby and saw lots of fish..
Also, for nearly 0 dollars you can catch a bus to 7 mile beach and pick a spot near a beach side restaurant, rent some loungers and hang out; the beach is beautiful..
I agree with Jamaica and want to have a planned tour their if you decide to do something.
Cozumel has a national park that we have not been to but hear lots of good things about it. Would be good for all your age groups. We have done a river rafting a few years ago and they are enjoyable; as you sit back on a raft and they pole you down the river and narrate some of the fauna and flora along the way...
Im sure you'll get lots of help from this site.
One thing as many will mention is to select a reputable outside tour group if you do not use the cruise line as the line will not wait for you if you miss the departure time.
We have been using outside tour groups for a while now and have had no issues and usually end up with a smaller group which makes for a more enjoyable day.