Hi Will,
If you're only talking about European tours then i guess all the capitals are the obvious contenders.
I would do the 3 capitals tour however it is simply not long enough- 12 days, 4 in each city would be perfect. The ones i would be interested in would be:
Paris, Berlin, Rome, Zagreb, Budapest, Vienna, Prague, Warsaw, Stockholm, Copenhagen, Oslo and Tallin.
Guess you have a lot of work to do hehe. Are there any that JY have in mind to produce?
Thanks