You can get fairly quick meals, even full ones, at many brasseries and cafes, and Paris doesn't lack for food on the go either.
The French often make a snack out of a simple baguette, torn off in hunks as they walk. Most shops will put together a simple sandwich for you to take away (à emporter)—or assemble the ingredients yourself for a picnic.
Paris's greatest street food is crêpes, sold at sidewalk stands and from store windows.
They're best when cooked fresh on the spot for you, but in touristy areas (Jardin des Tuileries, I'm looking at you here), crêpe stands often make up stacks in advance and merely reheat them on the griddle when you approach. Blech. Try to avoid these.
The cheapest, and in my opinion best, crêpe is the classic au beurre et sucre (with butter and sugar)—although you may prefer Nutella (a hazelnut-chocolate spread) and banana.
You can also get savory crêpes, such as a ham and cheese. That and a cup of hard cider make for a perfect quick lunch.
A dessert crepe. (Photo by Robyn Lee)