I remember a real smart alec in a session in the Elsinor in Whitby many many years ago - Can't remember what he was playing but whatever it was, it was in a key that most people had difficulty joining in with. I can't remember properly (It was early 80's!) but I think it was Ian Goodyear who joined in on English concertina, and then proceded to play through the same tune in every key - To a huge round of applause:-)

I guess the English concertina would be a pretty good choice for sessions but, as I don't play one, I don't really know!

DtG