So, the travel bug has hit me. We leave for Italy next week and already I am pondering future trips. I am doing some very preliminary research into a possible 2 week trip to Europe with 2 girlfriends next summer. We are thinking we would fly into Amsterdam (there are often good deals on flights from Canada) and spend 2-3 days there, and also go to Paris for 4-5 days. That leaves us with about 5 days or so somewhere else. Any ideas? We were thinking about Nice or Marseille but they seem to be quite far away, especially from Amsterdam. We would like to include some time in the country. We will likely be travelling by train though, so it can't be anywhere too far off the beaten track. One other option we had thought of was Brussels (not in the country, obviously, but looks kind of interesting). Any suggestions?

It depends on what you're looking for, but all these are convenient stops between Paris and Amsterdam -- Brussels, Antwerp, Bruges, Gent, The Hague, Rotterdam, Delft, etc.

Paris-Marseille is only 2 hrs 25 minutes by TGV from Paris..if you're going to be in Paris anyway. Otherwise, stick to Benlux I suppose if you are really concerned about getting back to Amsterdam

You might consider an open jaws itinerary, flying into Amsterdam and then flying out of an airport in France.
Sometimes, open jaws itineraries aren't that much more expensive than regular ones.

From Paris, it would be nice to visit one of the regions of France. Depending on your interests, Provence, Normandy, the Loire or Burgundy would each be a nice 5-day trip. If you will be doing all of your travel by train, the Loire is probably the easiest to navigate.

London is linked to Paris directly by rail these days.
That would complete a splendid trio.

I can second tropicaldan's idea of London having just done this trio. We started in London, took the train to Paris (it broke down halfway), train to The Netherlands and flew back to London. These three completely different cities made a great trip.

<<Brussels, Antwerp, Bruges, Gent, The Hague, Rotterdam, Delft, etc.>>

or detour east a little bit - - Cologne, Aachen, Maastricht, Liege/Luik, Louvain/Leuven - - bring you back to Brussels (gateway to Bugge or Gent)

Best wishes,

Rex