Can I use the Navigo Easy to go to La Defence (Grande Arche) from Gare Du Nord?
There's no direct train from Gare du Nord to La Défense / Grande Arche, but you can go from Chatelet les Halles to La Défense.
You can take Metro line 1 from Chatelet les Halles to La Défense using Navigo Easy with Metro tickets. We can't use the RER A to do the same trip with Navigo Easy since the Easy card cannot be loaded with RER tickets.
If you decide to put a Navigo Day Pass on a Navigo Easy (with at least zones 1-3 coverage), then you could use RER A from Chatelet to La Défense.