For the retreat, I'd pack any personal items you need for an overnight stay. Plus any charging cables for your device(s).
For staying in the lodge, you'll by sleeping on a twin mattress on a bunk (plenty of bunks - everyone will get a bottom bunk). A sleeping bag and pillow work best. But you can also bring sheets and blankets. Bring your own towel, washcloth, etc. Flashlight.
Comfortable shoes for walking or moving (if you're going to jog, play softball, frisbee, etc.) A book to read (reading lamp). Snacks. Playing cards or other games for free time in the evening. Refillable water bottle and/or travel coffee/tea mug.
Bring layers in case we get cooler overnight weather.
And bring an expectation of having a great time - because you will!
Bob Dinan
(Past attendees: am I forgetting anything?)