Amelia Island is thirteen miles long and about two miles across at its widest, and it is not homogenous. Understanding its geography is most of what you need to plan well.
The basic layout
The Atlantic is on the east side, and the beach runs almost the whole length of it. The Amelia River and several thousand acres of salt marsh are on the west side. A1A — called First Coast Highway on the south end and 8th Street in town — runs north to south. Fletcher Avenue runs along the ocean side.
The island's only city is Fernandina Beach, which occupies the northern third, and whose downtown historic district sits on the river, not the ocean. That single fact catches out more visitors than anything else about the place.
The southern two-thirds is unincorporated Nassau County: resort communities, golf, and progressively emptier beach the further south you go, ending in the undeveloped state park at the tip.
North to south
Old Town Fernandina — the original Spanish plat of 1811, north of downtown, with Fort Clinch State Park beyond it at the island's northern point.
Downtown Fernandina Beach — the fifty-block historic district on the river; dining, shopping, history and the marina.
Fernandina Beach oceanside — the residential and hotel strip along Fletcher Avenue, with Main Beach Park and the numbered beach accesses. Five minutes from downtown, on the sand.
Summer Beach — oceanfront condominium and resort development, including the Ritz-Carlton.
Amelia Island Plantation — the large planned community on the south end, now largely branded as the Omni resort, with golf, villas and marsh-side trails.
American Beach — the historic community founded in 1935, between the resort areas.
The south end — everything below Sadler Road broadly, running down to Amelia Island State Park and the Nassau Sound bridge.
Yulee and the mainland — across the bridges: the chain retail, the supermarkets, the airport route, and where a good deal of the island's workforce lives.
What this means for your trip
Distances are small but real. It is roughly fifteen minutes from the state park at the southern tip to Fort Clinch at the northern one, without traffic. There is no public transport to speak of, and the island is too long to walk end to end — but each individual district is walkable, and the Amelia Island Trail gives cyclists a paved route along part of A1A.