I had a similar experience some years ago at an early music summer school. We were working from a book called "The Division Flute" which dates from about 1720. The idea is that you improvise over a "ground" which is essentially a bass line in long notes - typically about 8 bars, usually one or, at most two notes to a bar. The group as a whole played the ground and each member in turn had to improvise a line over the top. It was very stimulating and got me interested in the possibilities of improvisation. I often used to sit at home and start a melody off, then let it take me where it would. I haven't done it much lately, I perhaps have spent too much time learning tunes, so maybe I should start again.

The book itself contains sets of variations played by well known professionals of the day and was written for amateurs who lacked the confidence to produce their own variations.