2 weeks will go quickly. I would consider an open jaw flight where I hit one area, buy a one way ticket on a low cost carrier to fly to another area, and then return from there. An example:
- Fly to Vienna
- Stay 3ish days, then train to Prague for another 3 days
- Fly to Brussels for a day or 2, hit Brugges then down to Paris for another 3-4 days
- Drag yourself back.

Another area worth considering is Italy, which hasn't yet been mentioned.

I'm envious already!