Menthe à l'eau is a drink mostly for French children, the thought of which can send people of a certain age into reveries. I discovered it the summer I was a high school exchange student in a small town in France. At the café or before lunch you might have un menthe à l'eau while the grownups had their apperitifs (although sometimes we did get to have the apperitif), or in the afternoon after bicycling around in the hot sun you could make it at home with store-bought syrup - maybe serving the role that lemonade served for American kids before Big Gulps ruled. But oh how sophisticated I felt ordering un menthe à l'eau at the café in the nearest big town!
All the pictures of menthe à l'eau that I could find are bright flourescent green, which was the color of the store-bought syrup; however I was given to understand by my "family" that the final drink was meant to be a very lightly tinged green, or else it was a sign you had put in too much syrup.
I have a mint plant growing as part of my "kitchen garden" outside my front windows, and I was thinking of what uses I might make of it for summer refreshment, and of course menthe à l'eau was on my mind. I found this recipe at Epicurious for mint simple syrup. I made a half batch using 3/4 cup of mint leaves (packed), 1/2 cup of sugar and 1/2 cup of water, enough for two or three large pitchers. The syrup makes an icy mint flavored water that is delicate and not overpowering, which as a grownup I appreciate more. Not to mention that it's practically gratuit.