I was wondering if you have any advice to make it better and if you have any "must see" things to do in these places.
Lisbon - 4 nights including a day trip to Sinta
Madrid- 3 nights
Barcelona- 4 nights
Sanotrini- 5 nights
Athens - 5 nights
It's a lot of moving around for three weeks, but it could work if that's what you want to do. You'll need to fly on most legs of the trip (except Madrid to Barcelona) due to the travel times involved by train
You need to allow a full day to travel between each place, and you haven't mentioned how you plan to travel. To get to Santorini from Barcelona you will need to fly to Athens, then get the ferry, very time-consuming.
I would drop Greece altogether and spend time in France instead.
Paris, France - alot of sightseeing to do. IJffeltower, Notre Dame de Paris, Louvre Palace, etc.
Rome, Italy - also alot of sightseeing and good food!
Also the scandinavian countries are pretty interesting and the air you breath in over there is way more cleaner than in the US or other European countries.
Five places in three weeks is a lot. Remember you will spend a day each time you change locations.
Too many destinations.
