The cabin is very well equipped but here's a list of suggested items you might want for your trip

Camera!
Sunscreen
Phone Card (No cell service at the cabin or most of the Rock Creek Valley; excellent cell service outside the valley)
Rain Gear
Binoculars or spotting scope (if you have room)
Clothing for temps that can range from snow to 90 degrees
Fishing or hunting gear

Recommended Reading: Fly Fisher's Guide to Montana - Greg Thomas
There is a copy of this book at the cabin - very handy to read before fishing a MT river