Itinerary help- destination to add to Paris trip

How about Amsterdam? (about three hours by train). It feels like a small town despite its size, has all you listed. Are flights booked?
I would stick to France, but that is a huge gap in my own travel history, so hopefully others will make suggestions. I would look at what the high speed train options are. Lyon appeals to me.
Do you have a sense of the length of trip you'd prefer (for example, keep it under three hours, etc.)?

Do you already have your plane tickets? Asking to know whether you are going to fly home from Paris or if you want to do an open jaw plane ticket.

Any thoughts on Belgium (Ghent and Bruges) or maybe Eastern France? Many go to Alsace for the wine aspects but I enjoyed 3 nights in just Colmar without touring wine areas but really enjoying the Alsacian architecture, history and food of the area. both areas are easy to get to by fast train from Paris.

Any interest in WWII for your historical sites? The girls may not be in to battle history but Bayeux and areas in Normandie might be of interest.

If you wanted to stay in France, you could consider a week in Normandy and the Loire. Especially if you are willing to rent a car. Train to Caen to pick up the rental. Visit Bayeux and Mont St Michel before heading to the Loire for a few nights. Perhaps stay in Amboise and explore the chateaux, before dropping the car in Tours and taking the TGV back to Paris for your last night before flying home.

I suggest staying relatively close to Paris for other destinations.

1) Brugges and Gent Belgium and Amsterdam. Suggest one day in each city, then 4-5 days in Amsterdam.
2) Normandy for 2-3 days or the Loire Valley for Chateux (Stay at Blois or Tours).
3) Strassbourg, France and Colmar--https://www.roadaffair.com/best-day-trips-from-strasbourg/
4) If you are interested in WWI history, the Verdun for one day. Also, Luxembourg for 2 days.

I’ll throw Lyon into the mix, an easy 2hour ride on the TGV. Great food, quaint old city, terrific public transportation system, interesting museums and Roman ruins.

I’d recommend staying in the Loire Valley for the week. Amboise is a charming town, and visiting other chateaux, towns, scenery is fun, relaxing but not boring.

I really like Lyon. It's got enough to keep you busy for several days. However, I think it's not going to feel as different from Paris as a lot of (all?) the other options offered here. On a trip like this, I think I like the idea of more contrast.

You're traveling in summer, so I'll mention that Normandy and Brittany are more reliably not-hot than a lot of the other options--though if reliably warm, sunny weather were essential to me, I wouldn't necessarily bet on Amsterdam either.

With your time constraint, I would suggest staying the other week in France, spending that time in Alsace-Lorraine or the Loire area.

What sort historical sites do you have in mind? You can access those connected with WW1 by trains dep from Paris Est and Paris Nord.