These are all the places I've really loved in Paris. I'd be comfortable at any of them as a solo diner.
La Jacobine
59-61 Rue Saint-André des Arts, 75006 Paris, France
My favorite restaurant in Paris. Excellent classic homestyle French dishes, salads, amazing desserts, and the best hot chocolate ever.
You need a reservation - they don’t take them online, and hardly answer the phone. Just stop by and make one in person, you can usually get one for the next day or two.
Le Temps des Cerises
31 Rue de la Cerisaie, 75004 Paris, France
Some classic French and some more creative/modern food with great daily specials, in a neat old house, on a quiet street. Best to make a reservation, but they can usually accommodate with a wait.
Crepolog
21 Rue Neuve Saint-Pierre, 75004 Paris, France
Good savory and sweet crepes - you can make your own combinations.
Ose
3 Rue Durantin, 75018 Paris, France
Tiny place, tiny changing menu, and tiny open kitchen. Very seasonal ingredients and mostly simple French food done very well.
Cafe L’Arsenal
2 Rue de Birague, 75004 Paris, France
Classic corner French cafe buzzing with locals. Fantastic omelets!
Chez Rene
14 Bd Saint-Germain, 75005 Paris, France
Classic French restaurant, with excellent bœuf bourguignon
L’As du Fallafel
34 Rue des Rosiers, 75004 Paris, France
Overrun with tourists, but very good Falafel and Israeli food. You can dine in, or get it to go - the falafel sandwiches in a pita pocket can be eaten on the street.
Boulangeries:
Chez Meunier
18 Rue Rambuteau, 75003 Paris, France
A few locations in Paris - excellent Viennoiserie, great bread, yummy quiche/salads/sandwiches
Boulangerie LIBERTÉ
40 Rue Saint-André des Arts, 75006 Paris, France
Amazing croissants
Boulangerie St. Antoine
29 Rue Saint-Antoine, 75004 Paris, France
Good Viennoiserie
Boulangerie Pâtisserie Sylvain Marie
12 Rue Coustou, 75018 Paris, France
Go for the bread and Viennoiserie, not the pastries.
Sweet Treats:
Richart
27 Rue Bonaparte, 75006 Paris, France
Chocolates and Macarons
Chocolat Illèné
2-4 Rue Tholozé, 75018 Paris, France
Chocolates
Pierre Hermé
Several locations
Macarons
Berthillon
31 Rue Saint-Louis en l'Île, 75004 Paris, France
Ice Cream
Amorino
Multiple Locations
Ice Cream