Simple good food with great attention to detail is always a win in my book. Nanashi does just that: simple, healthy, fat-free Japanese Bento and great fresh juices. They have a lot of Vegetarian options too, which are difficult to find in Paris in general. You can get your bento box with fish, meat or vegetables, with spring rolls before or after and a carrot ginger juice on the side and you'll leave feeling much refreshed and ready for your next, high calorie high carbs Parisian meal or pastry.
Find them in Le Marais and stop by the Broken Arm around the corner for a coffee and a slice cheesecake afterwards.