Formal:
* Une soirée à Paris: This is the most literal and formal translation.
* Une soirée parisienne: This means "a Parisian evening," emphasizing the Parisian atmosphere.
Informal:
* Une soirée dans Paris: This is more casual and can be used for a night out in the city.
* Une soirée à la parisienne: This is similar to "une soirée parisienne" but emphasizes the Parisian way of life.
You can also use the specific day of the week if you'd like, for example:
* Un jeudi soir à Paris: This translates to "a Thursday evening in Paris."
Ultimately, the best way to say "an evening in Paris" depends on the situation and your desired level of formality.