Similar to "fanciful" or "imaginative":
* Capricious
* Fanciful
* Fancyful
* Frivolous
* Impish
* Playful
* Quixotic
Similar to "unpredictable" or "eccentric":
* Erratic
* Fickle
* Impulsive
* Spontaneous
* Unconventional
Other synonyms:
* Amusing
* Charming
* Delightful
* Eerie
* Fanciful
* Idiosyncratic
* Lighthearted
* Whimsical
The best synonym depends on the context. For example, if you're describing a person who is often unpredictable and impulsive, "erratic" or "fickle" might be a better choice than "fanciful." If you're describing something that is imaginative and charming, "playful" or "lighthearted" might be more appropriate.