Always take a jacket of some sort, even if in the summer. I once had to stop on the way to a festival in Indiana in mid-August and buy a jacket when an unseasonable cold snap sent temperatures to nearly freezing.

And always take wet weather gear of some sort. Golf umbrellas are great. Decent plastic parkas that take up little room are readily available. And take an extra pair of shoes to wear while your main pair is drying out. Even a pair of rubber flip-flops beats wet soggy shoes.