General synonyms:
* Traditional
* Standard
* Usual
* Customary
* Common
* Ordinary
* Typical
* Established
* Accepted
* Mainstream
* Normative
More specific synonyms:
* Conservative (in terms of ideas or behavior)
* Old-fashioned (in terms of style or approach)
* Conservative (in terms of style or design)
* Orthodox (in terms of religious or political views)
* Formal (in terms of attire or behavior)
* Classic (in terms of style or design)
Antonyms for conventional:
* Unconventional
* Original
* Innovative
* Avant-garde
* Unorthodox
* Revolutionary
The best synonym for "conventional" will depend on the specific context of your sentence.
