Eric Kayser has pastry & croissants for 1,70 euros; basic sandwiches & salads to eat there or to go.
Their breads are good. There is one off Place Vendome in the 5th, I think address is 14 rue Monge, Metro station is Maubert Mutualite. They are open for breakfast & lunch.
Le Bougnat, small bistro (French cuisine), 28 rue de Saintonge near Nortre Dame; meal average price range $16-$24.
Best tip is to order the plat du jour, a glass of house wine, and "une carafe d'eau, which is a free pitcher of water. Bread is free. Tip is included on the bill, "service compris).
Salads, quiches, crepes, and plain omelets are the basic fare on a lunch menu.
I don't know your budget range but as you walk around, read the menus. They are posted.
Make a note of name & address for later.
Other suggetsions are buy a bottle of wine from the local shop. Enjoy a glass in your room before dinner. Buy a pastry from a shop and take it to go & eat oustide or back at the hotel.
Go to Galleries Lafayette department store. Eat in the cafeteria cafe upstairs and then go to the top and out onto the roof for free views of Paris! *Make a picnic and have it in one of the gardens, along the river, or on the pedesterian only bridges.
There are many Jewish delis in the 5th, they're closed on Saturday for the Sabbath.
I think you will have fun discovering how many places suit your tastes and budget.