The Montsouris park is one of my favourites in Paris, also because of its pretty surroundings. Whereas the northern Parc Buttes Chaumont is more in fashion amongst the young and hip in town, I prefer the calm and sweet Parc Montsouris, which was built in the same era as its northern brother.
Both parks are not the classic symmetric, controlled, French kind of parks like the Jardin du Luxembourg or the Tuileries – although beautiful, too – but rather have a romantic, pseudo-natural setup.
Another advantage is that lawns can actually be used to take a rest upon, something that is strictly forbidden in the Luxembourg (and where it still amazes and amuses me how park guards whistle as soon as a naive foreigner tries to get on the grass).
As currently it is not exactly the season for lying on the grass, the park is still worth a visit because of its pretty landscape architecture and the park-like neighbourhood. Just next to the park I enjoy having a look at square Montsouris: a very cute, village-like, little street with actual houses. A very similar street can be found in Montmartre, except that this one is not on a hill.
When you take the exit towards the south and cross the boulevard you will see the Cité Universitaire, a large green area that houses many international students in buildings of different kinds of architecture, each building representing a country, making me regret that I never actually studied in Paris.