It occurs to me that in music, as well as in other hobbies, you can get obsesseed with having more gear/gadgets/things.
SO...
when going to a session, I try to keep it to the essentials. In my gig bag I have:

-guitar (picks are in attached pick holder so techinically part if the guitar
-spare strings
-harmonicas in various keys
-harmonica rack,
-guitar tuner,
-song books/notes,
-pub prob (see other post on Mudcat for the latter)

Yikes, looks like too much when I write it down but everthing is used on a typical night (well maybe 4 of the 7 harmonicas).