Picnicking in Paris is the best! Although you are not allowed to sit on the grass in some parks, there are still many spots to picnic. Grab a seat on the grass of the Champ-de-Mars, along the edge of the Seine or on one of the pedestrian bridges such as the Pont des Arts. For a simple lunch or dinner just pick up sandwiches at a nearby boulangerie or create your own with a baguette, meat from a charcuterie and cheese from a fromagerie. Cojean, a French sandwich shop with locations all over the city has healthy and delicious to-go food options. You can also pick up ready-made sandwiches and picnic supplies at most grocery stores, particularly larger chains such as Franprix or Monoprix.
P.S. Here is a list in English of picnic spots in Paris and here is another list in French of parks that have picnic tables.
