After spending ages standing in line to Notre Dame and thereafter spending hours exploring everything there is to see in the cathedral, you can use a moment or ten of relaxation.
One idea can be to buy one of the delicious lunch menus in a bakery (boullangerie). You can often find an offer with sandwich (half a baguette with filling, ham & cheese, smoked salmon or egg and salad), a drink (a juice, half a litre of water or a coke) and a desert (a small berry pie, a piece of chocolate cake or flan). The price is often no more than 5-8 Euro, which is the lunch budget for many office employed. It is common that the companies give "Ticket Restaurant" to their employees as a benefit along with their pay. They can be used in restaurants, at the bakery and in some grocery shops.
When the lunch is secured, then go to the side of the great cathedral and find the park located to the right of the main entrance. The park is located between Notre Dame and the Seine. It's a nice park, which extends all the way to the tip of Ile de la Cité. There are playgrounds for the kids, tall shadowcasting trees, benches and wonderful green grass.
It is a nice spot in the middle of Paris where you can relax for a while and enjoy the bright sun and warm summer weather, while you're free of the noise and smog from the cars. You'll notice that many local go to the parks when the weather is warm and have a little lunch picnic before they return to work.