I saw this article on Yahoo and thought that someone might benefit from it. This is certainly something that travel agencies do not tell you about. So the top 10 cities are:

1. Barcelona, Spain

2. Rome, Italy

3. Prague, Czech Republic

4. Madrid, Spain

5. Paris, France

6. Florence, Italy

7. Buenos Aires, Argentina

8. Amsterdam , Netherlands

9. Athens , Greece

10. Hanoi , Vietnam

And the original article is here.