Best seafood is at Fin Bistro. Small, so get a reservation early.
Best sushi is at Mangos with Chef James. New "kitchen" chef too. Eric is doing a good job.
Bistro Soleil if you like French and mussels. Their Petite Soleil in the Olde Marco Shoppes has the best breakfast and a great lunch, especially if you want to sit outside.
If you're a fried chicken person, head to the Isles of Capri to Pelican Bend. A real Keys feel.
If you want a local yokel down and dirty place try the Sand Bar. Good happy hour and the best Buffalo wings here. Dolphin Tiki has picked up their game too with new owners. Open air on the water and relaxed.
Sunset celebration at Quinn's is fun. That is at your resort.
Lee Be Fish is quite good. Fish shack style food.
Verdi's is a good choice for a higher end resto.
Cupcake place in the little retail area near Lee Be Fish can make a cute 16th BD presentation for you.
I've been going to Marco for years but the food is never really first rate. For that you would want to drive to Naples. Not that bad a drive and worth it.
Ethnic ain't gonna happen on Marco. This is a midwestern snowbird area into meatloaf, fried grouper and crap wine.
It is changing, but at a glacial pace.