Suggestions for December Travel?

With 8 or 9 days, skip the "few other countries", as you barely have enough time to sample Italy. I'd suggest the classic Rome-Florence-Venice route. Frequent fast trains connect these cities, and they are overflowing with "romantic, fun, good food, history, and some nightlife". Fly to one Rome and out of Venice, or reverse. With 3 nights in each city, you have at most 2 1/2 days in each one.

Since you are in the Italy forum, I will confine my comments to Italy, which is wonderful in December! It's not warm, but the sites and museums are less crowded and the weather can be very nice. With only 8 or 9 days, you really only have time for two places, such as Rome and Florence. There's not enough time to do "a few other countries." If your travel is over the actual holidays of Dec 25 & 26, you will encounter some closures. Starting Dec 8, the Christmas lights come on all over Italy. Simply lovely to walk around in the evening and see the displays in the historical centers.

Someone else will have to handle nightlife. :-)

Although we did this over 17 days in February & March, and it included southern Italy, why not this, for 8 nights:

Fly to Paris, stay 3 nights
Train to Lucerne, Switzerland (4.5 hours), stay 2 nights
Train to Florence (~ 6 hrs), stay 3 nights
Fly out of Florence

It's somewhat of a whirlwind to be sure. But you'd get to see three countries in eight nights, trains are very efficient in Europe and fun in the winter. If before Christmas, you'll get to see how all these countries decorate for the holiday. And you can easily stay at hotels within walking distance of the train stations in Lucerne & Florence.

Enjoy your planning!

I get why some people don't want to try to do "too much", but I am not one of those. I think you should do as much as possible. The last time I was in Italy (2 year ago) I was there in December, and you get to see the Christmas markets and there are no crowds. The trains throughout Italy make travel easy, and if you really want to make good use of your time, consider an overnight train. Since Paris is our favorite city, I would suggest:

Day 1 arrive Paris
Day 2 Paris
Day 3 Paris
Day 4 Fly Paris to Verona (because it has easy train and plane access)
Day 5 train to Florence
Day 6 Florence
Day 7 train Florence to Rome
Day 8 Rome
Day 9 leave Rome for home.

I don't get the above comment.
Why would one fly from Paris to Verona to take a train to Florence? Especially considering that there are no direct non stop flights from Paris to Verona in December.
Why not fly directly from Paris to Florence with Air France or Vueling (or even to nearby Pisa with Easyjet)?

I like Jay's plan and I agree with Roberto's comment.

Go south if you don't want to see the big cities of Italy. Weather is better the further south go. Amalfi Coast, Calabria, or even an island, like Stromboli or Capri. You could go to Malta to see another country.