@manneca: from Gare du Nord you can also take Bus 43 direction Neuilly-Bagatelle until Gare St Lazare. The stop at Gare du Nord is directly in front of the station, it’s a 20 min trip and the stop is direct at the Hilton. It’s easier then the Metro (no steps), during rush hour maybe it’s crowded and slowly.
From the Roissybus stop at the Opera I would just walk (if it’s not heavy raining) it’s a short way.