Now that's what I call a tough question. It's like asking what is the one ride to see and experience at WDW - it depends so much on what you yourself like. For travelling - are you into cities or nature, the mountains or the beach? The beaten track (which "everyone has done" - the Canary Islands and Kos for example - or undiscovered terrirory - now with the EU enlarging, how about Estonia, Hungary or Slovenia? Food, culture, scenery, histroy or people? Do you go for cheap digs with lots of socializing with the other people at the local pub, or are you more the George V and Ritz type of person?
It also depends on how and when you're planning on doing the trip, and for how long. I assume that by "later this year" you mean sometime in the summer.
There's been lots of good ideas thrown in, but I'd like to add a few. Estonia, Poland, Hungary, the Czech Republic, Slovenia and especially Croatia have a lot of beautiful history, scenery and people. How about Tallinn, or Krakow, or Prague? Budapest or - one of my favourites - Dubrovnik? If it's frying on the beach you're in to, anywhere along the Croatian coast would be great. The Turkish coast also is wonderful. There's still a number of places in Spain, Italy (such as Sardinia) and Greece that are not overrun by the charter package crowd.
Something more rustic? Walking around central Spain, hiking the mountains in Norway or Austria, or norhern Portugal are fantastic.
Food? Gee, that's tough. But I'd rent a car and go just about anywhere in southern France, or Spain, or Italy - but away from the tourist coasts and the big-draw cities. (Although I'd try to get to stay in Venice overnight, when all the day-trippers have gone.)
I could go on and on - but space is running out, and I haven't even mentioned St Petersburg or Moscow, Copenhagen or Berlin ...