The highlights of the island for us was the beach, great shelling, and the resort we stayed at Marco Island Beach Resort was worth the price. Had a great time fishing the mangroves with my father. My son and wife enjoyed shelling on the beach. Whole family enjoyed miniature golf in Naples.
Quick Tips:
I would suggest booking local "backwater" fishing charter over deep sea fishing, more time to actually fish only took 30 minutes to get to good fishing grounds. Caught snook and mangrove snapper, and a couple of sea trout.
If you like to be in the center of the action, Naples is probably a better spot for you. Naples is around 20 minutes away from Marco Island, so if you like a more secluded beach, Marco Island is the ticket.
Best Way To Get Around:
You will need to rent a car unless you plan on spending all of your time at the resort. No real public transportation apparent. Grocery store was within a five minute drive along with a number of restaurants. Most major activities required a reasonable drive.