I'm at a disadvantage on some of this because I haven't been either to Marco Island or Clearwater Beach in years. My impression is that they are very similar, so maybe you'll have a little overkill on the Gulf Coast tourist beach vibe.
One dining suggestion I would offer is any Columbia Restaurant, anywhere you find them. For your trip, there is one on Clearwater Beach, and another on St. Armand's Circle in Sarasota (the side trip for lunch I was going to recommend). Columbia is an old-school Spanish/Cuban restaurant based in Tampa, and their food has always been exceptional...especially their seafood. The other thing you want from Columbia is their legendary "1905 Salad." Best salad ever, and I often get just that for a light lunch.
I believe
@Deb & Bill lives in or near Marco, so maybe she can provide better info about that area.
Sanibel and Captiva are nice...but pretty sleepy. Shell hunting is a big deal there, as is the
Ding Darling National Wildlife Refuge (managed by the US Fish & Wildlife Service, not the park service). Ding Darling is currently partially closed -- outdoor spaces are open, visitor center and all tours are closed.
We've been to Sanibel/Captiva twice in the last 3 years, and I am "Sanibel'd out." We actually stay at South Seas Resort on Captiva...boring (to me).
If you drive from Marco to Miami on US 41, rather than I-75, I would recommend two stops -- neither of which are available from I-75.
The first would be the
Shark Valley entrance to Everglades National Park. ALL South Florida NPS sites are currently closed (they love them some "administrative leave!"), but it's possible they will reopen by the time you visit. Shark Valley has a great naturalist-guided
2-hour tram tour that gives you an immersive 15 mile ride into the real Everglades to a 60 ft observation tower with views across the expanse of the Everglades. This is WET season, and within a couple of weeks the water levels will have risen significantly, so you'll see less wildlife than you'll see in Dec-Jan. But you'll still see some alligators, herons, egrets, ibis, hawks, etc. You will probably not see seasonal birds like some of the herons/egrets, spoonbills, etc. You may see endangered wood storks because there is a big wood stork rookery further east from Shark Valley.
The other stop I'd recommend would be for an
airboat tour. I recommend
Everglades Safari Park because their tour area is more scenic than other rides, and they have the best safety record. That's where we used to take VIPs when I worked at Everglades if we couldn't take them on our own boats for some reason. Everglades Safari Park is also closed by the NPS (they are regulated park concessioners and their tour area is actually
inside the park). If they are still closed, my second choice would be the Miccosukee Indian airboat rides at the Miccosukee Cultural Center immediately west of Shark Valley.
In Miami, if you stay in Sunny Isles Beach you will be in the heart of a highly-developed tourist spot. If you're ON the beach there, you will be on a great beach, with real ocean and real waves. You will be some distance from South Beach (12 miles, but may take an hour to drive) and downtown Miami (+/- 16 miles).
It's a shame you are only going to be in Miami one day. There is so much to see and do here, and we haven't even talked about the Florida Keys yet.
But you're coming at a bad time when many things will still be closed or heavily regulated, so make the best of it and come back and see us when you have more time.
