On a budget, Appian Way (Via Appia Antica) still earns its price.
Free (bike rental ~€15/day)
Walk or cycle the original basalt cobbles past tombs, ruins, and umbrella pines on a road that's car-free and green on Sundays — it feels worlds away from the city crush. The catch: it's long and spread out, so you'll want a bike and a plan, not a casual stroll.