Spend your sixth day exploring the historical sites and castles of Naples. Start with a visit to Castel dell'Ovo, a seaside castle with panoramic views of the city. Next, head to Castel Nuovo, also known as Maschio Angioino, a medieval castle that houses the Civic Museum. Explore the Royal Palace of Naples, a magnificent palace with opulent interiors and beautiful gardens. Don't miss the opportunity to visit the Certosa di San Martino, a former monastery that now houses a museum showcasing Neapolitan art and history. End your day with a stroll along the scenic Lungomare promenade.