L'arche de la Defense has finally been reopened to the public after 7 long years of work. 15 euro to take the lift to the top, for a great view down to the Arc de Triomphe. There is a restaurant, as before. There used to be a small art exhibit up there and I hope that will continue. Easy to get to La Defense by metro or bus.
The rer a line runs to the La defence district and a subway line runs parallel to it also
Very easy to get to
One transport warning - you can use a standard Paris metro ticket to get to La Defense by the metro, but not by RER.
This is the biggest fine trap on the Paris system. Arrive by RER at La Defense with a t+ ticket and you can't exit and are very likely to be caught and fined 35 Euro on the spot for riding on an invalid ticket.
The year before the Arche closed for its extended period, we went to a marvellous Christmas market on the esplanade. It was huge, taking up almost all the space on the esplanade. You could find everything, from jewellery to sausages. I hope this market will be back again.
Yes, wo, I have been to the one on the Champs (les Champs?) and it is great, too. Lots of mulled wine vendors!