This is Parc des Buttes-Chaumont, the first of several parcs we can visit in Paris.
It is in the 19th Arrondissment with views to Montmartre that look pretty great.
Here’s a clip from The Lonely Planet on this parc.
One of the city’s largest green spaces, Buttes Chaumont’s landscaped slopes hide grottoes, waterfalls, a lake and even an island topped with a temple to Sibylle. Once a gypsum quarry and rubbish dump, it was given its present form by Baron Haussmann in time for the opening of the 1867 Exposition Universelle. The tracks of the abandoned 19th-century Petite Ceinture railway line, which once circled Paris, run through the park.