I will be arriving 1 or 2 days ahead of my teenage niece in June so I will to need to go CDG and back again in the same day from central Paris to pick her up. I am trying to figure out the best way to do this. Do I just buy RER tickets (2 for me one for her) or get a Mobilis 1-5 for me? Is there another option? Thanks for any suggestions you can make.