I had such a good time, this morning, in parc Montsouris, in the south of Paris.
The park, created under Napoleon III, is named "Montsouris" because, at the time, the place was crossed by the Bièvre river and it was crowded with rodents (amongst which "souris" (mice).
The park is quite big (15 hectares). There are around 1400 trees (some are more than 100 years old), many species of birds, ducks, swans, fish... There are different areas where children can play, there are poneys, a merry-go-round and large lawns where parisians like to enjoy picnics.
Honestly, it's a very pleasant park.
Parc Montsouris - 2 rue Gazan - 75014 Paris