General:
* Magical
* Enchanted
* Mythical
* Otherworldly
* Dreamlike
* Whimsical
* Ethereal
* Surreal
* Fantastic
* Imaginative
* Fictional
* Fictive
* Unreal
* Extraordinary
* Unbelievable
* Impossible
* Wonderous
* Marvelous
Related to specific aspects of fantasy:
Magic:
* Arcane
* Spellbinding
* Mystical
* Sorcerous
* Enigmatic
* Supernatural
* Occult
Creatures:
* Mythological
* Legendary
* Fantastic
* Beasts
* Monstrous
* Dragon-like
* Fabled
Settings:
* Medieval
* Ancient
* Forgotten
* Lost
* Exotic
* Uncharted
* Unfamiliar
* Wonderous
Themes:
* Epic
* Romantic
* Dark
* Gothic
* Humorous
* Tragic
* Morally ambiguous
* Philosophical
Style:
* Lyrical
* Evocative
* Descriptive
* Vivid
* Intriguing
* Captivating
* Compelling
The best adjective to use will depend on the specific context of your writing.