Plaza de Marte is a small, lively park on the edge of Santiago de Cuba's historic centre. It is less formal than Parque Céspedes, but useful for getting a feel for everyday central Santiago: benches, shade, traffic around the edges, and locals passing through at all hours.
It works best as a waypoint rather than a destination in itself. Stop here while walking between the old centre and the streets leading toward the Moncada Barracks or the city's museums.