When I do a Jam session I usually don't rely on covers. I'll write a basic 12 bars blue riff and go off on that, everyone gets a solo spot, both guitarists,Drummer,Bassist, Keyboardest, and vocalist. Something I learned in Berkeley California at a Bar called Blakes where every Monday night the houseband (Blues) would play...then anyone could come up and jam with the band.
I'll do the same concept with my band...it hella fun...and it's a great way to get tight together while having a blast.

On the same note it can be done with rock n roll or heavy metal....country too for that matter.