3 days left in Europe, need advice on where to go.

This is a classic example of when an open jaw ticket would be the best option. Into Madrid and home from Athens. Depending on the time of year it could be a couple of plane changes to get to Madrid. You are covering some distance. Or you could split the distance and come home from Rome.

There is no logical way but flying. If you can change the return portion of your ticket to be from Athens, then that makes most sense. Otherwise, spend more time in Madrid (looks like you don't have much Madrid in the plan, yet) with daytrips to Toledo and/or Segovia. Or if you see a good flight from Athens to Barcelona, instead of to Madrid, you could have a couple of nights there.

Use kayak.com to play with options for flights that work best. Rome, Nice and Barcelona come to mind as likely to have frequent flights to Madrid. From Barcelona the train to Madrid is a good option as well.

Thanks for the suggestions, I am definitely considering Rome, Barcelona or somewhere else in Spain. I also thought Prague bur I think there won't be a many flights back to Madrid.

If it were me, and if I had to go back to Madrid, then I would simply go back to Madrid when the Greece portion of the trip ends and spend time in Madrid itself. As mentioned already, you could do a daytrip or two from there. Since Segovia and Toledo have already been mentioned, I’ll add El Escorial for consideration. Thing is, Madrid is worth two full days all by itself. Happy planning.

Thank you so much!!