Used to be just:

dance cards
full water bottle

and in the pocket of the bag that holds the box that holds the index cards:
name badge
pen
pencil
tin of pastilles (candies to moisten mouth)
dance tokens (not used at gig, just live with the cards -- used for visualizing how a dance works)
Human-a-tone nose flute (because it's small)
notepad
a couple of tissues
tampon

Nowadays, I add:

Shure wireless headset mic set

And sometimes (if I'm not in too much of a rush getting out the door):

folding barstool-height chair - standing for 3+ hours is not as fun as it once was, but I like to be "up" at standing height

Mac laptop computer with dance database + extension cord

This last I've carried a few times, but I'm finding it more trouble than it's worth. "Callers Companion" database software is nice for setting up a set list and displaying each dance in a large font (now that my eyesight is reaching that stage...). But, if the program needs to change on the fly (which it often does), the cards are much easier to shuffle through to find what I want. If I called more often, and had more of the database in my head, then maybe I could pull them up quickly enough from the computer. Even if I don't carry it, I use it for helping plan my evening.

~ Becky in Long Beach
(for contra dance calling gigs)