Where to go after 6 nights in Paris

Take the Eurostar to London. You could fly home from there, too.

Thanks. that is a possiblity. Any place else?

Are you definitely flying home from Paris? If so, that limits you to places fairly close, so you can get back. In that case, I'd suggest London or Amsterdam or Brussles/Bruges. If you can fly out of another city, then find one with a direct train, go there and fly home from there. Venice, Barcelona and Madrid all have night trains, if that timing works into your schedule. I'm naming cities, since visiting the countryside requires lots of traveling around, which you said you want to avoid. Staying in one city with lots of options of activities minimizes that.

I have done the following: Paris to Amsterdam (fast train, about 3 hours), Paris to Trier, Germany (about 3 hours, 1 change) and Paris to Brussels (about 1.5 hours) and they were all great but if I had limited time, I would probably go to Belgium. From Brussels, it's a short hop to Ghent, Antwerp or Bruges so you could enjoy more than one city with your available 3 nights.

Thanks for the suggestion. Yes, we would fly home from the place we visit after Paris. We have been to venice on a prior trip. Which do you like better? Madrid or Barcelona?

If you are interested in Barcelona or Madrid, I'd suggest that you just read up on both of them and choose the one that sounds more appealing to you. I liked them both but preferred Madrid to Barcelona; I know that many people have the opposite preference.

If you fly, which will take up most of a day, you can go anywhere in Europe. For shorter trips not suggested so far, Burgundy is 2 hours by train, Lyon 2 hours, Provence 3, Alsace 2 +. That's all French, of course, but each of these places has a very distinct regional identity and flavor. I do not think you would feel as though you were still in Parsi with any of these places.

We spent 3 nights in Brussels after Paris. Fairly short train ride, and you can easily do a day trip to Brugge by train. We stayed at the Novotel by the square and it was a great location and nice hotel. Next to the train station.

I would choose London, Belgium (maybe 2 nights in Bruges and 1 night in Brussels), or Amsterdam if you are looking for another country. You have gotten other good suggestions if you are willing or want to stay in France.