Hotel Saint Quentin and Hotel Marciano are both only 2 minutes from Gare du Nord. You can actually see Hotel Saint Quentin as you step out from the station in a little street. They are decent budget hotels with reasonable rates:
http://www.marcianohotel-garedunord.com/
http://paris-hotel-saint-quentin.com/
The metro otherwise operates till after midnight, even on week days. There are people at all times, and you never feel unsecure.