My wife and I have been on a couple of occasions and have had great moderate priced meals at a number of establishments in the St. Germain area heading from the river towards the Luxembourg Gardens and the Pantheon.
Additionally, a nice dinner experience on the more expensive side is George's on the top of the Pompidou Center. The design is quite modern and a bit funky inside, but the outdoor patio with good views over the city is the place to try and be sat.
Finally, we had a most wonderful and romantic dinner on the Seine with a very good meal, music, which ran from just before sunset into the evening meandering past various sights. I believe we did ours with Bateaux Parisiens a few years back.
At sunset, the Pont des Art (wooden planked bridge just shy of the island with Notre Dame) gathers a fair amount of people to watch the sun set, hang out, play music in some cases and just relax. Grab a baguette, bottle of wine and enjoy!
Depends on how much you want to spend. If you want to spend stay at the greates and only hotel on the Champs de, The Marriott. It is trully wonderfull.
Your other choice is a great place, La Defense Ren., The General Manager, Wille Libert, is the best. You are 10 minutes from the Champs at a major savings.
The Metro is easy and cheap.
Good luck and have a great time.
Other Paris threads to check
http://www.rewards-insiders.marriott.com/photoAlbums/1101 La Défense, Paris, France
SusanL (Renaissance Vendome)
Thanks & CDG to Renaissance Le Parc Trocadero (Renaissance Le Parc Trocadero)
Paris! "Never better!" (Arc d'Triomphe trip report)
Surprise (discussion on Paris sights & hotels)
Hotel and Metro in Paris (hotel airport transporation options)