Long time local.
Key Largo. With the exception of one or two, no bad scuba/snorkel ops in the Upper Keys. They're all good, they have to be.
Consider Pennekamp Park for an all day excursion for the family. Scuba, Snorkel, Glass Bottom, (all dedicated trips) Kayaking, or just lounging around.
Eateries in the Upper Keys. Many of these places are living on past reputations and have "sold out" i.e. positioned for the masses. I see names that I are fair to atrocious in their delivery of food and service. I recommend-
Mrs Macs or Key Largo Fisheries Backyard for lunch.
Made to Order mm 91 or Harriet's mm 95 for breakfast.
For dinner, depending on your budget of course. Nothing in Key Largo. Most of us locals gather at Old Tavernier (Italian and local fish dishes) mm 91. In Islamorada I suggest Kaiyo's, Morada Bay (Next to Islamorada Fish Co), Lazy Days and Chef Michael's. In the Lower Keys my fav is Square Grouper on Cudjoe. Key West, all great esp Louie's.
Bahia Honda State Park has twice daily snorkeling trips to Looe Key. A nice and refreshing alternative to the daily gear burden.
Enjoy your visit, hope this was some help.