Marco Island suits couples who want water, space and a long dinner. It does not suit couples who want a night out. That distinction is worth making in the first paragraph, because it decides whether the rest of this page is useful to you or a description of somebody else's holiday.
What the island does exceptionally well comes down to orientation and emptiness. Crescent Beach faces due west into open Gulf, with no landmass between the sand and the horizon, so the sunset here is the real article rather than a glow behind something. Fifteen minutes offshore there are islands with nobody on them, and a chartered boat will drop the two of you on one for the afternoon. The backcountry behind the island is silent enough that a paddle at first light feels like a private event. And between May and October, when the city's sea turtle lighting ordinance keeps the beachfront dark after nine, the sky over the Gulf is better than anything you will see from a mainland resort strip.
What it does not have is a scene. There is no club district, no late bar strip, no walkable restaurant quarter you can wander until something appeals. Dinner is booked, driven to and finished reasonably early, and in the quiet months a good proportion of the island's restaurants close a day or two a week. The median age here is well above the Florida average, the winter population is heavily seasonal, and the atmosphere is residential rather than resort-town. Couples in their twenties looking for a night out should base themselves in Naples or further north and come to Marco for the day.
There is a useful counterweight, and it is four minutes past the far end of San Marco Road. Goodland is two hundred people, stilt houses and stacked crab traps, and on a weekend afternoon it turns into the loudest, least polished, most enjoyable thing within thirty miles. It is the exact opposite of the condominium side of Marco, which is precisely why a day that ends there works so well.
So the shape of a good couples' trip here is: mornings on the water, afternoons somewhere with nobody else on it, sunset facing west, and dinner booked. What follows is how to build each of those properly, including which of them are worth spending real money on and which are free.