A)  A large towel.  This can serve many purposes: it can be a
comfortable rug, a comfortable seat cover, a drape, a
blanket, a furniture protector, a hair cover, and so on.

B)  A model-release form (several, if several photographers may
be present).

C)  A piece of chalk and a piece of charcoal, for marking poses.

D)  A male G-string (simple, not “jokey”), or a plain leotard,
(or body-suit) in case complete nudity is forbidden for some
reason – or you find nudity inappropriate for the people you
are working with.  Male G-strings can be purchased at
specialty lingerie shops in most metropolitan areas; leotards
are available at dance supply shops.

E)  A wind-up timer, so you can time poses yourself.

F)  Teabags, instant coffee, or something to warm you up
during breaks, in case nothing of the sort is made available.

G)  A couple of “interesting things” to pose with: see 22b.

H)  A pocket mirror: to check on your looks, and to use as a
“prop”.