Working for an airline, I've been able to travel to Europe quite a bit...not all football specific but here's my favorite places in each country I've been:
Ireland - Dublin's cool, but you gotta drive the countryside and head west to the Cliffs of Moher, Cork, etc.
Scotland - Glasgow's great, and of course Rangers and Celtic are in town.
England - London is massive and cool, but in my opinion a bit overrated. I've driven out west a bit...Weston-Super-Mare is a cool beach town, not far from Bristol which I really enjoyed. I've also been down to Portsmouth a handful of times for Pompey matches. Fratton Park's old, but has charm.
Spain - I didn't get to explore Madrid too much, but enjoyed what time I Had there. I used it as a base and explored Zaragosa (beautiful city) and Toldeo. Haven't been to Basque country or Catalan yet.
France - Paris lives up to the hype. Parc des Princes is a cool stadium but it's so damn far from the heart of Paris. Thankfully the metro is great. Also get north to Normandy if you can. Incredibly powerful and moving.
Belgium - I've just done Brussels. It's a neat city, but I was a bit underwhelmed compared to other locations I've been. Great food though.
Netherlands - Amsterdam's one of my favorite cities in the world. Super easy to get to the city from the airport, and getting around is a piece of cake, including to Johan Cruyff Arena.
Germany - Frankfurt is boring IMO...but Munich is spectacular. Hofbrauhaus is great. Everyone wants to go to Oktoberfest, but the Cgristkindlmarkts are so much fun. Nueremburg is also a cool destination not far from Munich.
Switzerland - The Alps are beautiful! Didn't explore Zurich at all, but I drove from there to Liechtenstein, and then spent time in Lucerne and Basel. It's an expensive country, but one of my favorites.
Italy - My favorite country in the world. Rome, Venice, Florence, all stunning. I also spent a bit of time in Milan, but didn't really enjoy it too much. Lake Como isn't too far however, and worth the drive if you're in the area.
Austria - I haven't been to Vienna, but really enjoyed the day trip I took to Salzburg.
Czechia - Similar to Austria, I haven't explored Prague, but spent time in the western part of the country in Karlovy Vary, a cool spa town.
Slovenia - Ljubljana is a hidden gem. Quiet, beautiful, full of history. Lake Bled isn't far away either and is beautiful all year.
Croatia - Split is cool, but most people will head to Dubrovnik. It was a main filming location for Game of Thrones, which has increased its visibility.
Bosnia and Herzegovina - I only spent a day here, but Mostar is a wonderful ancient city. The roads are terrible, and sadly there is still plenty of ruins from the Yugoslav war.
Montenegro - The drive along the Adriatic is stunning. I only explored Kotor but really enjoyed it. Tivat and Podgorica are supposed to be great as well.
Ukraine - Sadly I think won't happen for most people for a while. Which is too bad, because Kyiv is absolutely spectacular. A bit tougher to get around but the city is so cool. I also took a day trip to Chernobyl from Kyiv, which may be my favorite thing I've ever done.
Russia - Went to Moscow for the 2018 World Cup. I wish I had gone to St. Petersburg as well, but we only had time for one match at the Luzhniki (Spain v Russia). Red Square is cool, and for the World Cup it was a Fan Zone and full of people from all over the world.