Never miss the chance to go to Barcelona,even if you don't go to Madrid,you should see Barcelona,I've been there last year,it was most enjoyful. And as King Drahcir said,you should go to Rome,there are lotsa museums,cathedrals,old buildings,lovers fountain(I just came up with that

) what I really love about it,under the hot sunlight,you can get free water almost every street

Well PAris is nice place,but these two should your priorities if you don't want to regret it.My two friends is taking this tour too.They will be seeing Amsterdam,Brussel,some of Germany's cities,PAris,Madrid,Barcelona,Toledo(Endulus Culture,must to see,you can buy katana there,a masterpiece one

) of course Rome Vatican(they are close to each other) Napoli,Milano... There are more to add actually but itt's up to the time intervall you'll be staying there(in every city I mean)
How do I know this much,I have been lotsa cities that I have counted up there,last year.We would like to buy interrail,this yeear from its advantages,but it was a little late.We will be seeing Prague,Amsterdam,Münich Brussel and some other cities this year