Yes, a horrible predicament. I have decided to not fully decide. My tentative plan is to go to Amsterdam, because it involves a train ride and not a flight, and the associated time and inconvenience that goes with that. Madrid is definitely still a possibility, as is Venice, though Venice is less likely due to the time/expense/travel logistics.

The Loire and Normandy are still definite possibilities as well, though I am much more likely to do this if my traveling companion extends her trip by a few days, and I would rather not do this alone.

I wanted to put in my vote for Venice, because you have a general interest and it's a remarkable city. And, I believe there is still a night train between Paris and Venice which I find to be a fun experience and a good time saver! You can't go wrong, so enjoy whatever you decide and give us a trip report when you get back.