Well anything will do to be honest.. if you just want to jam, you can have as much or as little structure as you like. Both of my flatmates are musical and on rare occasions we will sit around just playing stuff.. nothing in particular.. just "stuff"... So like Dave will sit there on the electric piano, playing some ballad and I will sit there with my electric guitar jamming out a solo..

If you are already an accomplished guitar player, you should be able to solo along to just about anything.. from fast techno to classical music. You remember those "101 Easy Guitar Songs" books that you probably bought one or two of when you first started? These are ideal for such jam sessions.. the music in those beginner books is usually very straight-forward and also quite flexible. They chord progressions in those books will give you a starting point that is not in some unfamiliar setting. For the most part you will know the song, and will most likely realise that you're really playing the proper chords since it's such a beginner level book... but it should have you jamming in no time at all.