Where would you add to Paris for 3 days?

Colmar/Strasbourg

Tours (or Blois or Amboise) and chateaux nearby along the Loire.

I agree with Colmar/Strasbourg. We also enjoyed our night in Nancy, but I think there will be lots to do in and around Strasbourg.

Another vote for Colmar/Strasbourg.

St. Malo with a side trip to Mont St. Michel and perhaps to Carnac for the neolithic stones.

Luxembourg, Trier, Metz, Liege. Dijon. Lyon. Tours. Lille. All of these are good, and should keep you busy.

Strasbourg/Colmar is a standard answer here, but it's not the best choice. Luxembourg has more to do and is prettier than Strasbourg. Trier and Metz are beautiful and more historic. Liege is often overlooked. Lille is really nice, especially if you're into antiques. Dijon doesn't get a lot of comments, because it's not on the main tourist line. Don't go where you pay for the privilege of being another tourist, unless there's something there you just have to see or do.

Alsace - Strasbourg and Colmar. Easy train ride from Paris, and both lovely places. We've stayed in both many times, but prefer now to stay in Strasbourg because there are so many day trip options from there. Colmar is a 20 min. train ride where you can go and spend the day. We rented a car in Strasbourg for a day and drove to all the little wine towns. It was an easy drive, or you can get wine tours from there as well. We jumped on the train to Gengenbach, Germany one afternoon and really enjoyed that. It is less than a 30 train ride from Strasbourg, and we went there and had a late lunch and enjoyed the German decor of the town. Strasbourg has great restaurants and La Petite is charming as is the cathedral.

Another good option is to stay in Paris and plan day trips. There are lots of great ones near Paris on the train and you don't have to move bases. For example, Senlis is a wonderful medieval town. Some others are Moret sur Loing, Crecy la Chapelle, Dourdan. There are amazing chateaux. Versailles of course, but also Chantilly, Fontainbleau, Malmaison, and half a dozen others. There is Giverny and Monet's gardens and home. Rouen which is a small city but has a very different architectural style than Paris. And Auvers sur Oise the town where Van Gogh spent his last days and is buried.

RH,
What janettravels said about day trips. No moving hotels,and soooo many places to visit. Did anyone mention Chartres as well?

If you do Chartres -- stay a night so you can experience the illuminations. We did this last spring and it was very much worth it. The last train to Paris doesn't let you do the Cathedral illuminations and get back to Paris -- we stayed in an old quaint hotel steps from the Cathedral.