Well, at least you're not doing one of those 14 days/12 Countries!!!
Seriously, here are my picks:
Spain: Segovia is my favorite city...that and Toledo can be done as a day trip out of Madrid (a real party city!). I also recommend Grenada and Barcelona, but I would do just two of the three (Madrid w/day trips; Grenada/Seville; Barcelona). Otherwise, you're moving around too much.
France: Chuck Paris and its pomposity and head down to Provence...gorgeous Mediterranean scenery, Roman ruins, castles, beaches -- you name it!
Netherlands: Amsterdam, and as many day trips as sound interesting to you. The entire country can be pretty much done as a day trip with its excellent train system. I enjoyed Delfft...
Germany: I thought the Heidelberg area was really cool -- tons of castles.
Czech Republic: I've only done Prague (it's worth at least a few days). Others on the site have mentioned other towns that are interesting (check the E. Europe section).
Italy: My favorite country. I love Venice (make sure you spend an evening there to "feel" it after the day trippers have left. The Amalfi coast is gorgeous...and in the area are the ruins of Pompeii and Herculanaeum, the Greek temples at Paestum. And of course, Rome, the eternal city...
worldwidemike