Wow, can't believe nobody's replied to this, so far. Sorry 'bout that...I've been busy trying to organize my own trip!
Here are my suggestions:
UK - Scotland's gorgeous...get up to the highlands and islands (Hebrides) -- they were my favorites.
France - Provence is my fave there...nice Mediterranean countryside, cool castles, hilltop town, relaxed pace of life.
Italy - Ya gotta see Rome and Venice, then my money's on heading south to take in Pompeii, the Amalfi Coast...
Greece - Wow...so much to see! Athens, the islands, you could spend months there and not scratch the surface.
Turkey - Istanbul (Constantinople) is one of the great "World Cities." Definitely worth a few days. And loads of history, like Greece.
Croatia & Slovenia - they're on my list...haven't been there, yet!
Slovakia - not sure what's to do here
Austria - Alps, Vienna
Hungary - Budapest is nice...not as cool as Prague, but nice.
Switzerland - Alpine scenery
Germany - scads of castles, friendly people
Netherlands - Amsterdam is much more scenic than people expect. Delfft is a classic windmill dutch town.
What types of things do you like to do or see? Are you into history? Hiking? Hanging out? The more you tell us about yourself, the more on target our advice can be!
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."