I would say Salam. It means peace. I am not Muslim either but if someone wished me peace I would wish them peace as well. If you are sincere I am sure they will see your sincerity also.
the response to assalamu alaikom: which means “peace be upon you” is wa alaikom assalam: which means “peace be upon you too”
Assalamu alaikom Arthur and welcome to the forum 
in fact the xact translation would be " & upon you be peace" (if that’s grammatically right) :^^: