General synonyms:
* Commonplace
* Trivial
* Unoriginal
* Clichéd
* Hackneyed
* Overused
* Mediocre
* Dull
* Insipid
* Tedious
More nuanced synonyms:
* Prosaic: This emphasizes the lack of imagination or excitement.
* Stale: This implies something has lost its freshness or originality.
* Derivative: This suggests something is copied from or closely resembles something else.
* Uninspired: This highlights the lack of creativity or passion.
* Unremarkable: This emphasizes the lack of anything notable or interesting.
The best synonym for "banal" will depend on the specific context of your writing.