As noted above, there are many:

Fly to London. Spend 4-5 days there.
Chunnel train to Paris. Spend 4-5 days there, including Versailles and, if you like Monet, Giverny.
Train to Lyon. Spend a day there.
Train to Cinque Terre / La Spezia. 1-2 days.
Train to Rome. Spend 4-5 days there.
Train to Naples. Spend 2-3 days there at Pompeii, Amalfi, Sorrento, Positano.
Train back up to Florence. Spend 1-2 days there. Maybe side tour to Pisa one day.
Train to Venice. 1-2 days there.
Maybe switch to car. Drive over Alps, stopping at Lake Cuomo, San Bernardino, Lichtenstein. All this can be done in 1-2 days.
Continue to Munich--1-2 days. See Garmisch / fairy tale castle along the way.
Drive to Strasburg for a day.
Head back toward Paris, driving through Metz, Reims (cool cathedral) and Verdun (WWI site). Maybe side tour to Luxemburg.
If you really want to stretch to a month, drive north to Brussels / Bruges, then back to Calais
When done, train back to London to fly home. Or open-jaw it and just go back to Paris and fly home from there.