I relish dining out solo but get so few opportunities to do it. The theatre of a good restaurant can be a very entertaining spectacle that is sometimes best enjoyed when alone. Of course, you must insist on a strategic table or place at the bar where conversations can be overheard, and the clientele checked out. Much better than reading a book.
Pilgrim has read my mind (as she usually does) and offered many of the same places that I would have recommended. Especially Pierre Sang Oberkampf (there are several Pierre Sang restos but the one labelled Oberkampf is the most fun and has the best—and very interactive-- counter seating overlooking the kitchen). Dinner is a no-choice 6-course tasting menu (modern French with Korean inspirations, sometimes verging on fusion) but they will improvise if you let them know when you book if you have any allergies or aversions. And a stunningly good price/ quality ratio. Easily walkable from most parts of the Marais or hop on the #96 bus from rue Vieille Temple in the Marais to the Parmentier-Oberkampf stop. I echo onz’s feelings about Korus for a solo.
To Pilgrim’s and onzieme’s mostly very good lists I’ll also add:
Restaurant Diggity on rue St Maur in the hip 11th… usually, a very convivial ambiance and bar seating but a little caveat: most of the clientele are trendy 30-somethings and you could feel out of place (but you won’t be treated as if you don’t belong); https://www.diggityparis.com/
Le Mary Celeste on rue Commines @ rue Froissart in the 3rd/ Haut Marais… cocktail bar with eats, some bar seating, lovely terrace when weather is good, always some American tourists up for a chat; https://www.lemaryceleste.com/
La Grande Brasserie on rue Bastille in the 4th (just across the street fro, the well-known Bofinger brasserie)… no bar seating but very sparkling ambiance and a somewhat stylish Parisian clientele rather than the usual brasserie collection of fleshy French from the provinces and middle-aged tourists attracted by the decor (but that could quickly change now that tourists are returning to Paris), excellent trad cuisine slightly updated and served with flair (special mention for the joue de boeuf à la bourguignonne), usually excellent people-watching; https://www.grandebrasserie.fr/
GrandCoeur in a courtyard off the rue du Temple in the Marais/ 4th… very good restaurant “theatre”, waiters are very personable and flirty (but I suspect you are the wrong sex), a bit overpriced but not crushingly so; http://www.grandcoeur.paris/
For a splurge, Pavyllon on avenue Dutuit in the idyllic park-like section (between place Concorde and Rond Point) of the otherwise horrible Champs Elysées… a Michelin 1-star in multi-starred chef Yannick Alléno’s mini-empire, lovely setting, lots of bar seating and interaction with the kitchen (but in good weather, the terrace is perhaps a better choice), very cossetting service. https://www.yannick-alleno.com/fr/restaurants-reservation/pavyllon.html