wow - i love every single place i visited the 3 years i lived in europe ... sigh, i want to go back! LOL! my faves:
paris - i actually lived there and am TOTALLY biased

but - i think it's just the most wonderful place on earth!
southern france and corsica - we did a week in southern france (staying in a small town called eze sur mer) - did lots of traveling around, went to monaco at night, etc - then took the ferry over to corsica for 3 days ... very fun trip
croatia - i LOVED dubrovnik - such a beautiful, cool city ... we also spent a couple of days at the plitvice lakes national park (so so pretty!!) and the paklenica national park ... this was also an amazing trip
prague - i think prague is just amazing - one of my friends and i spent a week there and loved it ...
if you want to be a bit more adventurous - i loved
morocco and egypt ...

but would suggest going with a tour or someone who knows the area well ...
venice is a great short trip ( we went for a long weekend ) ... we stayed here:
http://www.hotelexcelsiorvenezia.com/ and it rocked - it wasn't actually in venice - but a short water taxi trip away - i loved those water taxi trips

very romantic - heehee
one of our favorite hotels EVER is this one
http://www.villadeste.it/en/13/home.aspx (villa d'este) in
lake como - it's really expensive though! LOL
ok - etc etc etc ... we also did several trips to spain - they were our 'golf and beach' relaxation trips, lots of club med trips which are super fun for families, a trip to portugal, i went to octoberfest and the running of the bulls, london A LOT (one of my best friends lived there), ALL.OVER.FRANCE, ireland ... maybe that's it ... there's so much i saw, but also SOOOOOOOOO much i didn't .... if you want any more info on any of the places i mentioned - let me know ..i LOVE to talk about it! hahahahaha ... although - i was there about 8 years ago - yikes! time flies!