We had one of our best vacation weeks ever at the Charter Club of Marco Beach several years ago! That resort is all 2BR timeshare condos, overlooking their pool area and/or the ocean with sunset views over the water. We mostly ate in so I can't comment on local restaurants though we heard others say they also drove up to Fort Myers for shopping and dinners.
During our week, we swam in the pool and the warm ocean. We rented cushioned lounges and a large umbrella for our use for the week to enjoy on the beach (they set them up daily for you). In the evenings, we could see schools of small fish and rays traveling up the beach in the shallow water. In fact, the entire beach region has fairly shallow water for a long way out from the shore - maybe 7-12 feet deep, w/lots of sandbars out in the ocean.
We borrowed a sunfish several times to sail up and down the coast of the island - they're free to use by guests at this resort but several vendors also rented them out on the beach. A lesson for beginners was free to anyone interested. They also rented seadoos and sea kayaks and they offered water-skiing.
There's a marina on the island that rents all kinds of boats for sport-fishing, bass fishing or pontoons for just relaxing. IIRC, a tour company offered a boat trip for a day in Key West. Our resort had info about this but we didn't go due to the cost for the 5 of us.
We drove about 30 minutes to Everglades City for an awesome airboat ride on a small, 6-passenger, very fast airboat through the smaller canals. It was a fun thrill ride well worth the fee. We spotted lots of wildlife out there - water fowl, alligators, manatees, turtles, etc.
We had brought fishing poles and crabbing traps on our trip. They're also available for rent or reasonably priced at a fishing bait & tackle sporting goods store on the island. We found two great places to fish/crab. One was in a public park at the far end of the main road past the Charter Club and the Hilton or Marriott (cannot recall which?). From there, we fished and caught all kinds of tropical fish (angelfish, tiger, puffer, small tuna, rays, others we could not identify) while watching manatees and dolphins pass by. Pelicans hung around nearby watching us and hoping for food. We also fished/crabbed under the main bridge to the island - nice and shady on a hot day.
Our resort also offered free use of bicycles and I've been told that there's a rental agency that offers them on the island too. If you're staying in a relatively quieter part of the island, with minimal traffic, this is fun.
There was some shopping in two different outdoor shopping centers we visited but it didn't seem like shopping is a big thing on this island.
The best part of Marco for us was every evening when we took sunset walks along the beach. It's pretty hard to find great sunsets over the water for us east coasters - really lovely. A few hotels along the beach have outdoor bars and cafés in back on the 2nd floor overlooking the water. You have to cross over the dune to reach these from the beach but the public is welcome.
Ask at your hotel or resort for ideas on things to do, if you want to be active. There's plenty there but it's not heavily advertised because it's not a touristy place. Have a great trip!
Also you can use the private Spa pool when you book a treatment. I believe it is the best resort on the island. There is not much else to do on the island but staying a fews days with all the resort has to offer is plenty. There are a few shops and I believe they are adding more. There is a plaza within walking distance with souvenier shops across the street and down the road apiece.
