Great shoes - for me Asics Gel-Nimbus and Brooks Ghost 3.
Nathan Handheld Waterbottle
Hat - some people hate them, but I can't live without my hats
Sunglasses - I would have a stack of them if I could! Currently I own a pair of Native's and a pair of Tilfosi's and a pair of Ryder's.
Body Glide - If you don't own it, you will soon discover why you should.
A vast array of moisture-wicking apparel - I happen to strongly favor Nike Dri-fit
And I think I'll be adding compression socks to the list very soon. I just got my first pair of CEPs, and I'm still in the discovery phase with them.
What are your "must-haves"?