In Berlin, the Tempelhof Airport in the middle of the city got turned into a park. Really great place to run, skate, or even ride a road bike. Grunewald is easily reached in a 20-30 min subway ride. I also loved running in Gesundbrunnen park, you can do a 2km loop but also do hill reps (with a nice view over the city).
In Buenos Aires there are some nice parks for running, but you have figure out how they’re connected so that you can avoid crossing the streets with lots of cars.
In Munich there’s some nice running in the English Garden.
I also loved running along the waterfront on Manhattan, not quite a park but you can run for long stretches without having to cross roads (great for marathon training).