I would say after many decades pondering the "easy tunes for a session" question, that the best answer is Winster Gallop and Cripple Creek. Now, these are both easy, and both very well known(and easy to join in). They may not be the tunes that experienced players necessarily choose to play at a session, but they are great for that virginity losing moment when the diffident beginner turns up, and somebody asks "What do you know? Start something up".Either of these tunes will cheer people up, show the beginner in a good light, and most imporatant it will get everbody else at the session instantly joining in. What more could you want?