Parque Verde do Mondego is Coimbra's easiest breathing space: a long riverside park below the steep old town and university hill. It gives you flat paths, water views, lawns, footbridges, and a gentler angle on a city that can otherwise feel like one long climb.
Use it between heavier sights. After the Biblioteca Joanina, the old cathedral, and the university courtyards, a walk by the Mondego resets the pace. It is also one of the better places to understand Coimbra as a river city rather than only a university city.