Glad you have your ticket. I'll keep my eyes open for any outdoor music events info. I can think of a couple of things, but have to check if they're on in August.
Edit:
Classical Music Festival at Parc Floral (on the eastern edge of Paris, at the Chateau de Vincennes -- line 1 Metro to Chateau de Vincennes and then a short walk to the Parc Floral). Admission to the park for the concert is 6 euros.
http://www.classiqueauvert.paris.fr/programmation
(look up the days of the week and know that "août" is the word for August and you should be able to tell what program is on what day. Keep in mind the French use the 24-hour clock so "16h" means 16:00 which means 4 pm.)
Generally Saturdays and Sundays at 4 pm, except for Sat. Aug 29th, when the program is at 8 pm. The series started on August 1 and goes through September 13.
Rock en Seine, in St Cloud (another near suburb of Paris, but on the northwestern edge). Domaine National de St. Cloud.
August 28-30.
http://www.sortiraparis.com/scenes/concert-musique/articles/64022-rock-en-seine-2015-le-programme-complet-et-les-horaires-des-concerts
Here's the English version of the Domaine National de St Cloud website with metro stop listed etc.
http://www.saint-cloud.monuments-nationaux.fr/
Outdoor summer film series at La Villette:
http://www.sortiraparis.com/loisirs/cinema/articles/50927-cinema-en-plein-air-de-la-villette-2015
Admission is free; it's 7 euros to rent a "transat" -- i.e. a beach chair to lounge in if you don't want to drag a blanket with you to sit on the ground.
All the films are screened in their original language with French subtitles.
The PDF gives the dates and titles of movies being screened.
http://lavillette.com/wp-content/uploads/2015/04/Brochure-CPA-Villette.pdf
Hope this will give you some ideas!