Here in the northwest corner of Montana, the temperature can swing pretty dramatically in a day. Keep that in mind and pack for sunshine-y days and cool nights, with a possibility of rain. In September, the average daily high is 68 degrees F, with it getting down into the 30's at night. So pack your flippy flops, but throw in some wool socks too! Again, this is NW Montana, so things like bug spray and bear spray are imperative. Below is a comprehensive list of things not to be without, as well as some extras to consider.
Necessities:
Journal/notebook
Hiking Shoes + Sandals or Water Shoes
Quick dry pants/shorts
Long sleeve/Base layer
Long pants
Jacket/Puffy
Extra socks (in case you get wet)
Hat
Swimsuit
Sunscreen
Bug Spray
Bear Spray
Headlamp and/or flashlight
Prescriptions
Water bottle or Camelback
*Snacks/bars
Camera
Toiletries
Sunglasses
Rain Jacket
Hand Sanitizer
*Some retreats provide meals and others don't - the retreat description lets you know what to expect. Snacks and beverages are always provided, but if you are attending a retreat that leaves meals up to you be sure to come prepared. A cooler, cookstove, cookware, utensils, FOOD, etc... We won't let you go hungry, but if it's not included, it's not included.
Extras:
Blanket for extra warmth or for use outside
Hat/Beanie
Outdoor Slippers/Comfy Shoes (This goes under necessity for me!)
Gloves/Mittens
Hand/Feet warmers (Shouldn't be necessary, but sometimes nice to have when you're stargazing:))
Day Pack (for hiking)
Wipes
Games/books/music
Instruments
Binoculars