Madrid Río is a real time commitment — fit it in only if it's a priority.
Allow 1–2 hours.
A model urban-renewal project: kilometres of greenway along the Manzanares, great for cycling, jogging and families, linking Matadero to the palace gardens. The catch is it's away from the tourist core and exposed under the summer sun, so it's a locals' park more than a sightseeing stop.