bon appétit is fine! Almost any English speaker would understand it, and probably appreciate the humour of a native french speaker saying it. Enjoy your meal is ok, or informally just 'enjoy'. I guess it may be more a cultural expectation for you to say something - we typically don't.

We actually just say "bon appetit." Otherwise "Enjoy" (if it's casual) or "Enjoy your meal" (usually only said by waiters) are both acceptable.

Thank you for your answers. I think I will try to use "Enjoy" and "Bon Appétit" and look for their reaction :-)