Acoustic jam! Singers, acoustic guitar players, bass (electric bass is OK!), mandolin, uke, banjo, percussion, harmonica, fiddle, accordion, piano (you are welcome to use ours) etc. And if you just want to spectate, that's fine too--no need to register for the event in that case-- just show up.
What kind of music? Anything that works: folk, rock, country, blues, cajun, irish, your own compositions, etc.
Bring a music stand and 20 copies of sheet music (just lyrics tabbed with guitar chords works best).