Bring your swim suit and bring a sweater and jacket. You just never know what kind of weather North Dakota will have those days! One day it can be 80 degrees and two days later there will be 5 inches of snow at your doorstep! I've lived there and seen it before.
Bring a bag large enough to hold a lot of belongings.
In the summer be sure to bring a lot of hot weather clothes. It is common for temperatures to raise into the 100 degree range here. In the winter make sure that you dress for subzero temperatures. Temperatures can easily reach 50 below here.
If it is summer, you need some warm weather clothing as temps can reach the 90's-100's. In winter you need really warm, wind-repellent clothing as temps, not even counting the windchills can reach 20 degrees BELOW ZERO.
If you visit the Peace Garden, make sure you have Identification with you. Part of the Park is in Canada. Trying to get back into the United States without identification is very difficult to say the least.