I always keep a mosquito head net in my bag. It's light, doesn't take up a lot of room, and can save your hunt.
Don't forget your binoculars.
Chapstick, surgical gloves for easy cleanup, extra glasses if you wear them, a trash bag, and a dose or two of Aleave or ibuprofen. A head or back ache can ruin a hunt.
That which does not kill me only makes me more irritable.