You can walk the oldest street plan on Amelia Island in Old Town Fernandina, the original Spanish town site on a bluff above the Amelia River, north of today's downtown. The City of Fernandina Beach calls it the last Spanish city platted in the Western Hemisphere, and the 1811 plat still shapes the neighborhood. It was laid out under the Law of the Indies, Spain's 1573 rules for new settlements: an orderly grid of evenly sized blocks around a central plaza, with lots measured in varas (about 33 inches) and sized as full peonias and half-lot media peonias. The first plan had about 13 blocks.
Old Town was listed on the National Register of Historic Places in 1990, and the city runs it as its own historic district. Its design guidelines protect the Spanish plat rather than any one architectural style, so the houses range from Low Country and Florida Vernacular to Spanish Colonial and contemporary. It's mostly residential, with only light commercial use allowed, and many streets and driveways are shell or spaced brick pavers. Things got quieter here once railroad man David Yulee persuaded residents to move the center of town south to Centre Street and the harbor.
The middle of it all is Fernandina Plaza Historic State Park, the site of the Spanish Fort San Carlos and its plaza. The visitors bureau points to it as a favorite spot for watching the sun go down over the river. The Pippi Longstocking House borders the plaza, and Bosque Bello Cemetery sits near the neighborhood's entrance on North 14th Street.
These are lived-in streets, so stay on the road and the plaza and leave the yards alone. If you own one of the homes, the Old Town area page covers what upkeep looks like inside the historic district.