For me it would be the harmonica intro to Doc Watson's "Lost John" which I think Merle played on "Doc Watson on Stage"--it's crossharp, and most of Doc's harp playing seems to be straight harp. The chords to "Alices Restaurant" would also work for me, since they're also the chords to "Take Your Finger Off It," which I used for a teachers' strike song, and which makes a great jamming tune. Or maybe if the audience would stand up and sing "La-ga-dee-yum, la-ga-dee-yum, la-ga-dee-yum, la-ga-dee-yum; Shoobie-doobie, oogle-ya-doobie, shoobie-dra-hah-hah..." and I could break into "Have some Madeira, M'dear."

--seed