Menthe à l'eau is easily one of my favorite things about France. It's name, "mint in the water" is perfectly descriptive. It is, in fact, mint syrup diluted in a glass of water. This beverage can be purchased at any side-walk café in France. Or better yet, you can buy a one-liter bottle of the syrup for roughly the same cost that a vendor will charge for a single glass at a café. The experience is essentially like drinking a bottle of Scope, only without the germ-killing burning sensation.
Menthe à l'eau is not only more refreshing than water, but it is also a good benchmark for how expensive a restaurant/café is. You can generally expect to pay between 12 and 15 French francs per glass at a normally-priced café. If it costs any more than 15 ff, then the restaurant/café is going to be a bit pricy. Since this drink is popular in France, and exists almost everywhere, you should not have any problem finding it listed under "boissons fraîches" (cold drinks), and then comparing the cost to between 12 and 15 ff.