The improvisation concerts at American Guild of Organists conventions are mind-blowing. Performers are given a tune and improvise a work (toccata, theme and variations, etc.) on the spot. Even knowing the tricks they're using—such as having a general outline mapped out ahead of time that can be used with any tune—I'm still in awe.

I got pretty good at improvising (and sightreading) when I was the primary organist at my church. Not having much time to practice (young kids, a different full time job), I did what I had to to get by.