campaign challenge, keeping things fresh

Not only do I think they are the more easily reached challenges, I enjoy the campaign and some of the challenges set (i.e Pot Kettle Black) illustrate new ways of simply playing the campaign. It wasn’t until the PKB challenge that I even realized you could shoot those things down.

I’ve been skimming Daft’s challenge guide (and many thanks for that), seeing the kinds that have come up. Regardless of the official daily challenges, what has been the more interesting campaign challenges to date? I’m interested in new ways to play the campaign (outside of speed runs).