Horsh Beirut, a wedge of stone pine forest at the southern edge of the city, is by far the largest green space in Beirut. It was closed to the public for decades and only fully reopened in 2016 after a long campaign by Beirut-area activists.
The park has gravel paths, picnic benches, and a remarkable density of birds in spring. It sits on the old Green Line and was repeatedly damaged in the civil war; the pines are slowly regrowing. It is open daily and free to enter. Combine with the nearby Beirut Hippodrome.