Which is a good place to visit for an overnight (accesible by train) close to Paris?

What would you be hoping to find/experience there? One person's "good" is another's "awful".

Chantilly
Chartres
Dijon
Versailles (RER)
St-Denis (metro)

Good for...what? What are your requirements?

Some would say Disney..I might say...Reims.

Mont St. Michel

Auxerre

Bayeus nice old town with great cathedral and queen Matilda's Tapestries dating from 1066 to commemorate the Norman Invasion of England launched from near here - and you have lots of mini-bus tours to the nearby D-Day Landing beaches and Collville-s-Mer American Military Cemetery overlooking the beaches where so many lying here died.

About two hours by train from Paris - a nice regional town to spend an overnight in.

There are hundreds of possibilities. Without more information from you about what you are seeking, we would be wasting our time offering up suggestions. What, exactly, do you want to experience?

And it's Bayeux, not Bayeus, in case you want to investigate.