I happened to catch this excellent Paris-by-night tour suggestion from Rick Steves' website, one that is worth passing along. Instead of going on a group bus tour, Rick recommends just taking a taxi!
If you live in Paris and will be hosting some out of town guests, or if you are planning to visit Paris in the near future, this sounds like a great way to spend about an hour one evening, visiting some of the most beautiful parts of the City of Light, at night! Rick even gives you the French you'll need to communicate with your taxi driver... and a map so the driver will know where you want to go.
What an easy, fun idea! On my last trip to Paris, we had dinner with a Parisienne I know, and she brought along a friend/client who had her own car (I think she lived in one of the 'burbs). On the way back to our hotel, they insisted on driving us around to some of the sites at night, so we went by the Louvre, the Arc de Triomphe, Place de la Bastille, and more. It was a lovely way to see the city and much more intimate than a tour bus.
So go out for a lovely dinner at a special restaurant (I recommend Le Procope in the 6th) and then grab a cab for a little après-dîner tour of the most beautiful city in the world. Then stop in one of the famous cafés (Flore or Deux Magots, also in the 6th) for a late crème brulée or chocolate mousse and an espresso. You'll have a truly memorable evening. Bon voyage!