deluxe
Especially luxurious, elaborate, and of high quality. It describes a version that goes beyond the standard.
Wir haben uns für das Hotelzimmer in der Deluxe-Version entschieden.
We chose the hotel room in the deluxe version.

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Especially luxurious, elaborate, and of high quality. It describes a version that goes beyond the standard.
Wir haben uns für das Hotelzimmer in der Deluxe-Version entschieden.
We chose the hotel room in the deluxe version.

The adjective 'deluxe' is invariable and is not declined. It always keeps the same form, no matter which noun it comes before.
Wir buchten ein Deluxe-Zimmer mit Meerblick.
We booked a deluxe room with a sea view.
'Deluxe' comes from French ('de luxe') and literally means 'of luxury'. This origin explains why it is not declined as a foreign word in German.
Das Menü war wirklich deluxe.
The menu was truly deluxe.
When 'deluxe' comes before a noun, it is often treated like part of a compound word and connected with a hyphen.
Im Kino gibt es spezielle Deluxe-Sitze.
The cinema has special deluxe seats.
simple / just
'einfach' and 'deluxe' are opposites when it comes to features, style, or quality. 'einfach' describes something plain, without luxury or special extras. 'deluxe' describes an especially luxurious and better version, often with more comfort or higher quality. 'einfach' sounds more practical and modest, while 'deluxe' sounds like something special and more expensive.

Für den Urlaub haben sie ein Deluxe-Zimmer mit Blick aufs Meer gewählt.
For their vacation, they chose a deluxe room with a view of the sea.
Wir haben ein einfaches Zimmer gebucht, weil wir nur eine Nacht bleiben.
We booked a simple room because we are only staying one night.