If it's warm (and it should be in September) and sunny, I'd check my bag(s), go to the Tuileries or Luxembourg Gardens, pull up one of those chairs that lean way back and are hard to get out of, put my feet on another one and lean back and enjoy the sunshine, fresh air and doze. Of course, make sure your money and phone are secure. If you don't doze, the surroundings are beautiful, so you relax either way.