The list is endless, depends upon what do you like. if you like to go the typical touristy way, then London, Paris are must. But, if you want to see the real European beauty, you could go to Austria (Vienna, Salzburg, insbruk). very similar to Switzerland, but much cheaper.
2 days in Paris.
2 days in Switzerland
2 days in Austria
1 day in Pisa
1 day at Rome
2 days in Milan
If have more time then visit Spain and Portugal
went in 2017
Different countries have different culture...I know its literally very tough to cover all the European countries at the same time.Nederlands, Belgium,Poland, Spain and Hungary, Switzerland and France - you should not miss these countries atleast...