Challenge: end the game in as few turns as possible, in a solitaire game. You don't have to have any points or anything. No conditions. Bonus for a strategy that can work quickly consistently instead of with just great shuffle luck.
I was playing around with this last night and kept getting stuck. Here's what I came up with so far, although I'm sure that it's far from optimal.
We all know than in Puzzle #1, you can have all 8 Provinces by the end of your 5th turn. But that requires perfect shuffle luck. Nevertheless, one of my first thoughts was that following that puzzle's strategy would still lead to a fast game without perfect shuffle luck. It doesn't.
So I also tried various things with Governor, because everyone knows that that leads to fast games. Nothing doing.
The best I came up with was this: Open Bridge / Fishing Village. Buy Council Rooms, Throne Rooms, King's Courts, and more Bridges and Fishing Villages. This allowed me to pull a mega-turn on turn 10 and end it (with all the Colonies, Provinces, and Duchies!)