For Shelling, I highly recommend “Treasure Seekers Shell Tours”. My Wife is a hard core Shell collector and they are by far the best. If you are up to it, they will get you out to some GREAT, rarely touched locations.
Consider an Airboat tour of the Everglades but make sure the water is up or the boat will be limited to the deeper canals. You will see some BIG Gators.
You can fish from the Naples Pier (not sure about license requirements).
Delonore-Wiggins State Park, Tigertail Beach, Barefoot Beach
Rent a Kayak or better yet take a guided Kayak tour.
CJ’s marina bar on Marco Island. The food is meh but sit outside.
Doreen’s Cup of Joe, also on Marco for breakfast.
Sanibel Island
Charter fishing but fishing has been shit in recent years.
There are a few sunset booze cruises in the area.
Visit the Bass Pro Shop
Bonita Springs Flea Market
Visit the Indian Reservation along the Tamiami Trail