If you only had one meal in Paris, it would be an absolute tragedy. For a restaurant I would recommend you go here or here, & for drinks go here or here, but really, really the only thing to do is to picnic. Trever and I usually spend 60% of our meals in Paris, on the side of Seine eating all the goodies we've picked up during our stroll through the city that day.
So, get your little self to the Isle St. Louis. Go on the Rue St. Louis en I'lle and stop at this Cremerie for cheese (side note, click on the street view of the building because the owner is giving the camera the bird, it's spectacular). The cheese guys don’t speak very good English so just pick something pretty. Then head next door to the boulangerie for some macarons and a baguette, or two if you are feeling crazy. Lastly, stop at this market to grab some grapes, peaches and Rose.
Then start meandering towards the west end of the island. Sit RIGHT HERE and eat bread, cheese and fruit and watch the sunset turn the city pink. Make sure to pack some wine glasses, or you can just chug (I mean, not chug, sip) straight from the bottle, which we do sometimes - we're classy people. This is the point at which the city will put it's little tallons in you and suck you in so you can never stop loving it. Forever you will feel desperate to spend the rest of your life right there, on the side of the Seine, eating cheese.