Can elected city council members be on the board of a non profit organization?

Yes, as long as it does not create a conflict of interest. A conflict could exist if, say, the non-profit provided some sort of service to the city for a fee, or if the city provided grant funding to the non-profit. Even in those situations, the person might be able to maintain both positions by disclosing the potential conflict and abstaining from votes where there was a conflict.