Requesting Trip Advice from European Savvy Travelers

EDIT: Just realized this might belong in a separate forum. Please move as appropriate, thanks.

------------

Thanks in advance for your time and advice.

Just booked my first Europe trip for me and my girlfriend:
2/28/13: ATL-ZRH (Zurich)
3/10/13: CDG (Paris) - ATL

I plan on spending first 3 nights in Zurich, last 3 in Paris, and 3 in between in a yet to be determined city. European fights from ZRH - "City X" - CDG are typically $200 or less, so I want to hit a third city (fast paced trip, city highlights).

My initial thought was Vienna in between, as my parents rave about it.

What are your thoughts? Bear in mind this is a winter trip!

Thanks!