Think of a key.
Thomas hopes to not have offended anyone with this post. No responsibility whatsoever is taken for any spelling or grammar mistakes, should there be any.

Play songs, improvise on jazz standards, blues it out, write your own songs, just jam on a chord progression...

The options are only as limited as you are.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.

Quote by Master Foo
“A man who mistakes secrets for knowledge is like a man who, seeking light, hugs a candle so closely that he smothers it and burns his hand.”

One of you come up with a riff, and the other comes up with something to go with it.
Guitar and bass sessions are the best, drummers are just awkward!
Well,this is what I usually do:
Play songs
Drummer plays a random beat,we choose a key to mess around with and have fun.
Ibanez SA160QM
Laney HCM10
Squier Bullet Strat
MXR Carbon Copy
Zoom Tri Metal
Modtone Flanger(mini)
Korg Pitchblack
Timtone acoustic