This lovely park stretches across the 24,5 ha area between the Eiffel Tower and the École Militaire. In the past, it has been used e.g. as private gardens or for military drills. Nowadays it is one of the best places from where to admire the stunning Tour Eiffel.
You can enjoy various outdoor activities here - e.g. have a picnic or play some ball games. Also, during many French festivals, the fireworks can be watched from here.
You may come across many vendors here selling mini Eiffel Towers and also beggars with various messages describing their (usually fake) sad life stories. Both these groups can be quite annoying but harmless.
Wikipedia https://en.wikipedia.org/wiki/Champ_de_Mars
Address 2 Allée Adrienne Lecouvreur, 75007 Paris, France
Coordinates 48°51'21.763" N 2°17'52.527" E