Even tho I have been going to retreats for years, and have even organized some, at my last retreat I forgot the most important thing-the power cord to my machine-and I wasn't the only one, luckily I had brought a machine for someone else coming the next day so I was able to use that & a friend coming the next day picked up the machine cord for me.
Most important-all cords, foot pedal for machine; surge protector power cord & extension cord. Usually we share irons because with too many irons we have blown the fuses of the room more than once.
And have fun, I love the retreats!!