One day is enough to see why people come back, provided you do not drive the length of the island more than once. The island is thirteen miles long, and the single biggest waste of a day here is doubling back.
The plan
8am — Fort Clinch State Park. Be there at opening. The park is at the island's northern tip, the drive in runs a mile under live oak canopy, and the 1847 masonry fort is the island's best single attraction: ramparts, galleries, kitchens, barracks, all walkable rather than roped off. Two hours for the fort; add an hour if you want the pier or the jetty beach.
Bring water — the brickwork holds heat and there is no shade on the ramparts.
10.30am — Old Town, briefly. The park entrance is at the edge of Old Town Fernandina, laid out on a Spanish plat of 1811. Fifteen minutes driving the grid and reading the markers, plus Bosque Bello Cemetery if the mood takes you. Almost nobody does this and it costs you nothing but the detour.
11.30am — downtown Fernandina Beach. Park once. Eight blocks of Centre Street in Victorian storefronts, the cross streets, the Fernandina Harbor Marina and the working shrimp docks at the bottom.
Lunch here. Counter-service seafood if you want to keep moving; a courtyard restaurant if you would rather sit.
If you have an hour and the weather is against you, the Amelia Island Museum of History is the fastest route into the Eight Flags story — eight national flags have flown over this island, more than anywhere else in the United States.
2pm — the beach. Now go east. Main Beach Park is the closest to downtown with full facilities: free lot, restrooms, showers, a playground. If it is full, Seaside Park is a mile south, or drive down to Peters Point on the south end, which has more room.
Two to three hours is right. The sand is wide and firm, and walking five minutes from the access gets you space.
5pm — one south-end stop, if you have the energy. American Beach and Burney Park are worth twenty minutes for the history: the community founded in 1935 so that Black Floridians could reach an ocean beach during segregation, with Florida's tallest dune protected behind it.
Sunset — back downtown, on the river. This is the part that catches people out. The island faces east: the Atlantic side gets sunrise, and the sunset happens over the Amelia River on the west side, downtown. The rooftop decks at the bottom of Centre Street are the spot; arrive well before golden hour.
Dinner downtown, walking distance from wherever you watched the sky.
If you only have half a day
Fort Clinch and downtown. Skip the beach. That sounds wrong for a beach island, but the fort plus a Victorian port town is the version of Amelia Island you cannot get elsewhere, and sand you can.
If you are arriving with children
Swap Old Town for more beach time, and put Main Beach Park in the middle of the day rather than the afternoon — it is the one access with a playground and food across the street.
What to skip on a one-day visit
The Amelia Island Lighthouse. Florida's oldest, but an active navigation aid on grounds that are not a public park, viewable only from a residential street. On a one-day trip it is a detour for a photograph.
Kayaking and Cumberland Island. Both are excellent and both eat half a day or more. Save them for a longer visit.
Practical notes
Parking is free downtown and at the city and county beaches, and Florida state parks charge per vehicle. There is no meaningful public transport, so this is a driving day. Festival weekends — early May and March especially — close downtown streets and invalidate this plan entirely.