Key West via the Key West Express (~3 hours to Key West, departing in season from Marco Island's Rose Mariana) or drive down yourself(~5 hours to Key West).
If you go to Key West and are staying overnight there, perhaps you would like to visit:
J.N. Ding Darling Wildlife Refuge on Sanibel Island (1 hour 40 minutes)
Naples Zoo (40 minutes)
Everglades National Park - Shark Valley (1 hour 15 minutes. Bike, walk or take the tram tour, about 15 miles round trip over pavement to view the park.
Bird watching and great photo opportunities at the Corkscrew Sanctuary (1 hour 15 minutes)
Boating tours and information on the Everglades (50 minutes)
Rookery Bay National Estuarine Research Reserve (17 minutes) Be sure to check out the Rookery Bay Naturalist Guided Tours boat and kayak trips.
Conservancy of Southwest Florida (33 minutes) They have a great Nature Center, von Arx Wildlife Hospital, guided kayak and electric boat tours as well as other events.
Naples Botanical Gardens (40 minutes)
Naples Philharmonic Complex (41 minutes)
Big Cypress National Preserve (1 hour 10 minutes)
Fakahatchee Strand Preserve State Park (30 minutes)
Collier-Seminole State Park (20 minutes)
Miromar Outlets (45 minutes)
Naples Outlet Center - Premium Outlets (20 minutes)
Sun Harvest Citrus (1 hour, 10 minutes, hopefully you can hit this on the way down, 20 minutes from (RSW) Southwest Florida International Airport, they offer free samples!
Deep Lagoon Seafood Naples (45 minutes) Lunchtime grilled fish tacos are a favorite 239-593-5555