The Appian Way (or Via Appia Antica) was built during the height of the Roman empire as an essential way of connecting central Rome to the region at large. What remains is a largely stone road (built in the 4th century BC) and a collection of ancient buildings that are fully worth a day trip via the Termini bus station.
Fifteen kilometers long and under natural park protection, there are vast numbers of ruins, catacombs, churches, and more, from full structures to simple burial mounds. The road begins at the Porta San Sebastiano, a gate in the Aurelian Wall, and follows into the countryside, providing access to highlights like the Church of Domine Quo Vadis (where St. Peter is said to have met Jesus), the massive and picturesque tomb of Caecilia Metella, and the many catacombs of San Sebastiano Church.
Walking and biking are the most popular ways to explore the area, with bikes available for rent at the park office. If you've already seen enough piazzas full of tourists to last a lifetime, but still have a craving for antiquity, then head to the outskirts of Rome for Via Appia Antica.