Similar meaning, but less formal:
* Waggish
* Jocular
* Humorous
* Playful
* Lighthearted
* Whimsical
* Bantering
* Joking
* Satirical
* Ironical (when used to convey a sarcastic tone)
More formal and negative connotations:
* Flippant
* Insolent
* Irreverent
* Mocking
* Sarcastic (stronger than ironic)
* Cynical
* Capricious (implies a whimsical and unpredictable nature)
* Frivolous (implies a lack of seriousness)
Other options:
* Clever
* Witty (if the humor is intelligent and quick)
* Droll (if the humor is dry and understated)
The best synonym for "facetious" will depend on the specific context of your writing. Consider the tone, the relationship between the speaker and listener, and the intended effect.