Sorry haven't read all the entries on this but I go to a festival every weekend from May to Sept and I find I need: This is like one of those drinking games...

Loo paper
air bed
my 6X4" thick peices of foam rubber
a quilt
an under quilt
my pillows
a night dress that looks like a dress when roaming to the loo first thing
torch
insulation to go under the air bed
oh and a tent!!!

there must be more but i can't think of it just yet.

Sal