The heavy hand of tourism is inescapable in central Paris and to a large extent you will be sharing your time and space with other tourists, not Parisians. But Paris is a surprisingly compact city and you can easily escape the tourist zones to sample a more authentic “vie parisienne”. Many tourists walk everywhere but that is very limiting and time-wasting. The métro system is very good but in a city like Paris nobody should cocoon themselves underground and miss the flavour and pulse of Paris on the surface. I for one prefer the bus or biking and you get a mini-sightseeing tour on the way. The bus system can initially seem very intimidating for foreigners but, believe me, it’s a cinch once you figure out the routes. The RATP itinerary planning function is a good start. When I am in an unfamiliar city I use Citymapper app on my phone/ laptop and presume it to be good in Paris as well. https://www.ratp.fr/ and https://citymapper.com/paris?lang=fr&l=fr (sorry, I use the French version… google to find English language version)
LouLou. The setting is fab, The food (Italian) not so much. If you can snag an outside table, probably enjoyable. If the weather is bad or they give you a table inside, probably disappointing. For alternatives, the seasonable terrace restaurant Les Petites Mains in the Palais Galliera/ fashion museum near the Palais de Tokyo/ modern art museum in the 16th is much better foodwise and comes with a view of the Tour Eiffel. https://www.lespetitesmains.paris/la-saison
No experience of La Boite aux Lettres that I can remember. It’s a little too close to the place du Tertre/ Sacré-Coeur ultra-touristy scene (which doesn’t make it necessarily bad). I tend to prefer the more genuinely charming (or at least less contrived) bits of Montmartre west of métro Abbesses. Some suggestions that will oblige you to experience the non-tourist side of Montmartre (in order of the quality of the cuisine): Chantoiseau on rue Lepic for excellent updated (lighter) classic French cuisine; Bistrot du Marquis on rue Caulaincourt; La Bascule on rue Durantin for an early light dinner of French tapas/ shareable small plates (no lunch, open 5pm to 1am, a popular hangout for 20- and 30-something Parisians so probably fun even if you don’t drink). Also, maybe afternoon tea and cakes at Terrass Hotel’s bar-resto with pano view. https://www.chantoiseau-paris.fr/ and https://www.bistrotdumaquis.com/ and https://www.labascule-abbesses.fr/ and https://www.terrass-hotel.com/restauration/bar-rooftop
Il Etait un Square is really just a hamburger joint and a not very convenient or appealing location. Not worth the trek and you can easily find other hamburger places that more easily slide into your tourist schedule. Just check this list of hamburger restos recommended by one of the local food critiques to find a place that fits your itinerary. https://scope.lefigaro.fr/liste/les-meilleurs-burgers-de-paris-55449620/ This site sometimes has a paywall. If so, let me know and we can figure out another way.
Not sure how Tuks Kebabs got on your list. Its kebabs are overstuffed with lots of unnecessary and taste-confusing stuff but probably very instagrammable. I only know the Canal St Martin location (takeaway), not the resto in the 13th. I much prefer the more trad kebabs in “La Petite Turquie”, a Turkish/ Kurd neighbourhood centered on the rue du Faubourg St Denis on the 10th …. favourites are Urfa Durum and Ozlem (on rue Petites Ecuries) for meat kebabs and Daily Syrien for falafel. It’s also adjacent to a hangout area, centered on Cour des Petites Ecuries, favoured by 20- and 30-something Parisians.
One bouillon is probably enough. My fave is Bouillon République because of its excellent price/ quality ratio, vibe and location on the fringes of the very enjoyable and trendy Haut Marais in the upper 3rd. If you want a more tourist-pleasing décor, Petit Bouillon Pharamond is an Art-Nouveau gem. Neither Bouillon takes reservations and the queues/ waits for a table can be long, especially between 8 and 10pm.
L’As du Fallafel. I’ll just copy and paste my remarks on another thread: “Speaking of lemmings, the mindless ritual of queuing up for “the best falafel” in Paris at l’As du Fallafel in the Marais mystifies me. I haven’t been to every falafel joint in Paris but enough to know that there are a dozen other falafel places that are as good or better. I even prefer Mi-Va-Mi (almost no queue, falafel better suited to my tastes, usually much more personable service) just across the street from L’As du Fallafel or to avoid the Fishermen’s Wharf-like scene on rue des Rosiers altogether. Paris has lots of other very good places for falafel”
Chez Janou. Can be lots of tourists and foreign expats and the food is good (not remarkable) but the ambiance is very enjoyable and convivial. Probably should remain on your list.
Robert et Louise. I haven’t been in a very long time. A 99% tourist restaurant and popular with organized tour groups. It’s very cutesy. I would delete but I’m not a tourist nor a sucker for the cutesy.
Aux Bons Crus. Very good… and a chance to explore the hip non-touristy 11th arrondissement. Yet, I prefer its sister restaurant Aux Crus de Bourgogne in the 2nd because of the somewhat better quality of food, better desserts. and usually very enjoyable, very Parisian vibe in a historic setting. Take your pick. http://www.auxcrusdebourgogne.com/
Closerie des Lilas. A has-been place exploiting its Hemingway associations and serving rather mediocre trad cuisine. The Piano Bar is, however, very sparkling and recommendable. For a meal or just hanging out on a sunny day on its people-watching terrace, my go-to in this neighbourhood is La Rotonde on the corner of boulevards Montparnasse and Raspail. Classic French cuisine done as it should be and a very enjoyable ambiance. https://larotonde-montparnasse.fr/
La Jacobine. Not a place that I particularly like but, because of its location in a very atmospheric cobblestoned passage, probably a very enjoyable tourist experience. It also gives you the opportunity to escape the 21st century for the 18th by exploring the adjacent Cour de Rohan, accessed from the Cour du Commerce St André, and other linked passages and courtyards. BTW, Chez les Libanais on nearby rue St-André-des-Arts has excellent falafel and will save you from feeling the need to stand in a very long queue at L’As du Fallafel in the Marais.
The pricey Le Vent d’Amour and the more trendy Belle Maison are both very good fish restaurants. While the food is probably better at Le Vent d’Amour, Belle Maison would probably appeal more to a pair of 20-somethings. To complicate matters, I’d also suggest Istr on rue Notre Dame de Nazareth on the fringes of the very trendy Haut Marais and, for a very casual and well-priced seafood/ oysters resto, Paris Pêche Sea Bar in the vibrant and very parisian Aligre quartier (great neighbourhood for shaking off the tourist dust) . https://www.istr.paris/ and https://www.paris-peche.com/seabar/
Chez Michel and its sibling Chez Casimir are keepers for the food. The setting around the Eglise St Vincent-de-Paul is pleasant but the general neighbourhood can sometimes be a bit sketchy.
Fish la Boissonnerie (despite its name, not a fish restaurant) is good but in the summer becomes almost like eating in a French restaurant in Seattle or New York. St Germain des Prés is one of the most popular hotel/ AirB&B quartiers for American tourists and the tone and demographics of Fish la Boissonnerie reflects that. Locals are more likely to eat at Huguette (fish restaurant) on rue de Seine. https://www.huguette-bistro.com/
I very much like La Grande Brasserie on rue Bastille in the Marias just off place Bastille. Newish so not yet discovered by tourists. So far, a youngish and very stylish Parisian clientele, but, given the huge surge in tourism that Paris is now experiencing, that could change quite quickly. Excellent trad cuisine in a sparkling setting. https://www.grandebrasserie.fr/ For the vibe, style, and food I’m also a big fan of the new-ish Lolo on rue de Châteaudun near Notre-Dame-de-Lorette. https://www.lolocaveamanger.fr/
I’m pretty sure that the lists suggested by other posters are not really appropriate for a pair of 20-something Singaporeans. Le Fooding is probably more relevant for the under-40s and has firmer hand on the pulse of today’s Paris . https://lefooding.com/ (English language tab at bottom of the page)