Two days to kill.......opinions?

You might consider taking the train to Bayeux or Caen to take a D Day beaches tour and visit the many tracts of hallowed ground. Bayeux is our favorite since it's smaller and has great accommodations. Worth the time IMHO.

Doug, Instead of 9+ hours on the train, I would check into flying.
Easy Jet has some pretty reasonable fares.

I'd also try to avoid the return to Paris altogether - can you switch to open jaws?

What time of year is the trip, and what sorts of things do yo like to do?

Fly back to Paris from Milan, no its not that comfy on the train, its not the Orient Express , lol
Look at Easyjet or Vueling, avoid Ryanair as they use far out airport . Sorry can't compute rest of your question, since you only have four days in Paris I would stay in Paris, my shortest visit was 9 days, and that was too short for me!

I'd also suggest flying - look at www.whichbudget.com to see which airline has the best deal for your dates (keeping in mind that you'll probably want to fly into major airports - CDG and Malpensa.) For Italy, I'd consider 2 nights in either Verona or Bologna. Both easy train trips from Milan Central Station. If you want more time in France - Lyon. If you like the idea of going to Verona, Bologna or Lyon you could fly there from Paris and then train to Milan.

What David said. Varenna on Lake Como, as I recall,is about a one-hour train ride north of Milan.

Take a budget air carrier from Paris to Milan, then spend your days up at Lake Como on the Swiss border. The scenery is absolutely incredible.