General Synonyms:
* Bizarre
* Eccentric
* Extravagant
* Fantastic
* Outrageous
* Preposterous
* Ridiculous
* Unconventional
* Unorthodox
* Wild
More Specific Synonyms:
* Absurd: for something completely illogical or nonsensical
* Far-fetched: for something highly improbable or unbelievable
* Flamboyant: for something showy or dramatic
* Grotesque: for something distorted or ugly in a bizarre way
* Ludicrous: for something that is silly or absurd
* Whimsical: for something playful or imaginative
The best synonym will depend on the specific context of your writing.