Le Boulanger des Invalides, where I intend to sign up for a meal plan.
Pastry and coffee served in same place, check.
Pleasant indoor and outdoor seating options, check.
Reasonable prices, check.
Croissant made with butter and noticeably superior to what would be found on a random corner in NY, check.
Non-croissant options available as well, check.
Open all summer (!), check.
Slight disadvantage: not anywhere near where I'm living, or will live, or any of the libraries. But if a dome-shaped attic apartment were to fall into my lap...