We have been spending 1-2 weeks on Marco Island over the holidays for the last 8 years at the Surf Club and Charter Club (very close to Crystal Shores), and these are a few of our favorite restaurants, in no particular order:
Speakeasy for Italian
Cocomo's for seafood
Mango's Dockside Bistro
CJ's on the Bay
Capri Fish House (on Isle of Capri) for seafood, cool tiki hut right on the water
Joey's Pizza (our favorite pizza on the island)
Marco Brewery for a sports bar environment, good food and large beer selection
Pinchers (I know it is a chain, but we like it for a lunch and is across the street from Crystal Shores)
Salty Dog (close to Crystal Shores, good shrimp tacos, bar food, sports bar)
Little Bar in Goodland, absolute best place for stone crabs; not much to look at, but have always had good food there, seating on the water
Island Gypsy on Isle of Capri (a great place on a nice day for outdoor seating on the marina, onion rings to die for)
And for an ice cream treat, Sweet Annie's is the best on the island, better than Beebe's and worth the drive.
Although the Snook Inn is always a popular place, I have been underwhelmed by the food and it just isn't worth the wait to get a table there. Also have had bad experiences with Nacho Mama's.
Kurt