I collect mint sets from The Netherlands, by year.
Furthermore, I try to get complete sets of each Euro issuing country, mostly through finding them in change. I'm looking for UNC, but occasionaly I first find a lower grade that I keep as hole filler, until a better grade comes by.
Just like CCG, In the beginning of the euro era, I also traded many coins with fellow collectors in various countries.
In the past I also collected by mint mark or minor design changes (e.g. Belgium, has changed the nation side slightly adn the european side has changed as well, when new countriess started issuing euro coins), but I gave that up, I only go for one nice set for each country, unless a country decides to do a significant redesign, e.g. becasue of a change of ruler (as we will expeirence this year and Vatican City hasa a couple of years ago).