Taking good photos is about more than just the camera.
What accessories are you sure to take with you?
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Good, comfortable footwear - many types of photography involve walking a fair bit, and you don't want your feet getting sore and being a distraction.
Similarly, suitable clothing in general - if there might be a great shot in the next field over, don't let fancy trousers stop you climbing a fence to have a look!
Oh and a friend - to warn in case that one last half-step to get the scene framed perfectly will result in a big splash. ;)
Some sort of calibration thingy to include in at least one shot per "scene". There are a number of different makes & models from different makers -- I like the Lastolite collapsible XpoBalance and the XRite ColorChecker Passport -- but anything that will help you set a white balance at shooting time and set white, mid and black points in post will help immensely.
Oh, speaking of Lastolite (as one brand, again, among many good ones), it profiteth not to venture forth shooting people without a reflector. I usually carry a collapsible white/soft gold model with me, even for "just walkin' around".
All good suggestions ... I would like to repeat the 'extra memory cards' thought.
I had a situation where I brought a single high capacity memory card, but the card had some kind of error on it and I was stuck. Luckily my wife had a spare, lower capcity, card with her ... so I could take some pictures (but I couldn't take them in raw+jpg the way I wanted).
gallon sized zip-lock bags
tripod (but only if I have a specific reason, usually this is left behind)