General synonyms:
* Clichéd
* Commonplace
* Ordinary
* Trivial
* Unoriginal
* Hackneyed
* Trite
* Stale
* Overused
* Insipid
More specific synonyms:
* Predictable (if the banality is due to a lack of surprise)
* Superficial (if the banality is due to a lack of depth)
* Tiresome (if the banality is annoying)
* Uninspired (if the banality is due to a lack of creativity)
* Mediocre (if the banality is due to a lack of quality)
The best synonym will depend on the context of your sentence. For example, if you're talking about a boring movie, "clichéd" or "uninspired" might be good choices. If you're talking about a conversation that's full of small talk, "trivial" or "superficial" might be better.