Spontaneous musicking with others in an unpredictable live setting, where we have a somewhat agreed upon foundation for the repertoire (e.g. jazz standards) and can be creative in the moment within that (i.e. jam sessions). IOW, play music for the fun of it.
It’s a bit of a challenge, given that open jam sessions can differ widely in terms of venue and participants. But there’s something, at least for me, appealing playing guitar in that framework, without worrying about catering to an audience or about making recordings.