Chère Madame Sushmita ¶
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Vendredi, Le 7 juillet 2023, Delhi ¶
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Bonjour Madame.¶
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Chère Madame Sushmita,¶

French date format = day of the week + cardinal number (except for the first of each month which is "le premier") + year, and then place
The letter salutation stands after this, so if you have one, you don't need to write "Bonjour"

Dans cette lettre, je vais vous décrire mon voyage ien France chez mon correspondant.

Je suis arrivé à Paris, France à le 5 mai et ensuite, j'allais à chez mon correspondante suis allé chez lui.

Since you're talking about France, it's obvious you're talking about the capital of France and not Paris in Texas, for instance
Days aren't introduced with any preposition => le 5 mai
"à chez" doesn't exist
"lui" to avoid repeating "correspondant"

Après rester poury être resté une journée, nous sommeavons visités le Sacré Cœur à Paris àde nuit et nous avons mangé audans un restaurant proche de celui-ci.

"après" is about an achieved action => past infinitive, "être resté"
Duration is almost never indicated by "pour", rather with "pendant/durant" or (very often) nothing at all, like here
"visiter" works with the auxiliary "avoir"
"AU restaurant proche" means there's just one restaurant which is close to the Sacré-Coeur

Ensuite, le jour prochlendemain, nous avons visité laes villes d'Avignon et de Carnac.

The next day = le lendemain
Avignon and Carnac, that makes two => les villes

À Avignon, nous avons visité le pont d'Avignon et let [fait une Bbalade ?] sur le Rhône.

Did you mean you sailed on the Rhône?
"d'Avignon" would be unnecessary here

EnÀ Carnac, nous avons visité les menhirs de Carnac.

All cities require "à" => à Paris, à Avignon, à Carnac, à Delhi, à Pékin, à Berline etc
Same remark here about unnecessary specifications

Ensuite, nous avons fait une randonnées de 6 km et avons eupris le Rrepas chez des amis de la famille.

to hike = faire une randonnée
you "take" a meal in French. It also sounds much more natural to use the appropriate verb for every meal (like "dîner" for "having dinner" for instance)

Le jour prochlendemain, nous avons visité dles jardins et dule château de Chambord etpuis avons ebu leun cocktail au château.

This is a specific castle and there's only one => LE château
"puis" ("and then") to avoid repeating "et"
"avoir" isn't used as much in French as it is in English: although "avoir un cocktail" wouldn't be wrong grammatically, but it would feel awkward. "Boire" would be far better
"LE cocktail" would mean a definite cocktail that you haven't mentioned before

Ensuite, je suis sorti pourfin, je [me suis rendu/je suis retourné à l'aéroport ?].

"sortir pour l'aéroport" wouldn't mean much sense in French: get out of what?
If you just mean you went to the airport to come back home => je me suis rendu/je suis allé à l'aéroport
You can even say "je suis retourné" since you've already been there (when your plane landed)
"enfin" would be the appropriate term for the a last element

Au revoir Cordialement,¶
Kushagra

"cordialement" is the most used and neutral letter salutation at the end of a message.