input on itinerary - 3days unplanned

If you are heading from Bordeaux to Paris, maybe consider staying for a few nights in or near Tours (Amboise or Blois would also be considerations). You'd be right in the heart of the Loire valley, with some beautiful chateaux (Chenonceaux, Chambord, Amboise, etc.). Also not far from Chartres and its stunning cathedral. You could even take the TGV from Bordeaux to Tours if you preferred not to drive (although having a car to travel around the Loire Valley is probably the easiest way to see everything). Bordeaux to Tours is about a 3 hour drive, and you'd be moving in the direction of Paris. And then just take the TGV from the Tours area to Paris.

Alternatively, if you want to stay closer to Bordeaux and then take the TGV from there to Paris, I'd consider going to Toulouse and other sites (like Carcassone and Albi). Toulouse is about a 5.5 to 6 hour train ride via TGV to Paris.

I'd either pick the Loire for the 3 nights and tour the great chateaux of the Loire with a car or stay in Burgundy for the 3 nights. We love the stunning little town of Semur en Auxois and after staying there for a night on a trip from Provence to Paris, we booked it for a week on a later trip and day tripped from our little cottage to towns in the region. You can see snapshots of Semur and a few of the day trips in my photo journal under 'Burgundy.' The area around Vezelay and Auxerre is also nice. We stayed at a little inn near Avalon for a couple of nights on the edge of the Morvan forrest and did some day tripping from there. The hotel is the Moulin des Ruats and it has a terrific restaurant that attracts people from the area as well as a nice terrace to sit and read alongside the little stream. We took some walks in the forrest from there as well. (and you can get trout bleu hauled up out of their creek while you await the meal; they keep fresh trout in the millrace)

janettravels44
www.janettravels.wordpress.com

I guess this is your Viking River Cruise ...
http://www.vikingrivercruises.com.au/cruise-destinations/europe/chateaux-rivers-wine/2015-bordeaux-saint-bordeaux/index.html
Perhaps a gentle 3 day journey ...
Day 1 Bordeaux to Tours
Day 2 Tours to Orleans via Loire chateaux (Chambord + Chenonceu)
http://chambord.org/en/
http://www.chenonceau.com/index.php/en
Day 3 Paris via Fontainebleu
http://www.musee-chateau-fontainebleau.fr/spip.php?lang=en

All good suggestions above. Just to make the choices harder, I'd add the Dordogne country around Sarlat for castles and caves and truffles and pate. Unless your cruise is taking you there already, but I'd be surprised if the Dordogne has enough water for a Viking cruise!

Rent the car in Bordeaux and drive it to Paris, maybe to Orly airport where you can shuttle and metro into the city. And pick just one of the suggested areas -- Languedoc, Dordogne, Loire (maybe +Chartres) to do it justice in the time you have. The autoroutes are fast and fun to drive, and a car will give you flexibility you need in all those areas.

I agree with you that Aix is too far given your timetable.

Chartres is a good substitute for Fontainebleau as you are heading into Paris ... if you are all shat out of chateaux.

I don't understand: Chartres is the Gothic cathedral, Fontainebleau is a palace. Very different.

I meant as a "substitute on your itinerary" not as "something similar to substitute".