Going to (Not Running a) Marathon

We're taking my parents to Marathon, Florida, this coming Friday. That's the mid-Keys, about 50 miles northeast of Key West. We'll be starting in Miami and driving down in Avis' finest mini-van.

I haven't been to Marathon since I was 19, during a spring break jaunt while visiting my brother (AKA Pepper Bro), who was living in Miami at the time. For some reason, he and a friend decided around midnight that it would be a great idea to drive down through the Keys. Those two days are a jumble in my head of early, early morning driving (so early that we ended up sleeping in beach chairs next to a canal and only through the kindness of State Trooper Friendly did we not get run off or ticketed for trespassing); sunburn; heat; sunburn; and a seemingly genuine offer from my brother's friend to visit him and his family in South America, which I politely declined. Oh, and sunburn.

Anyway, it's been more than 20 years since my last visit. So does anyone have any can't-miss restaurants or family-style attractions I should plan for? All recommendations will be duly considered with my gratitude.