Here are some accessories that I use for my Sony a7sii, but could work for any alpha a7, a7r, a7s mk1 or mk2 adventure shooting kit. Some, like the SDXC Memory Cards and a Screen Protector, are what I'd consider to be 'must haves', the others are nice to have.
64GB SDXC Memory Card
I know I read this somewhere before buying my camera, but in the excitement, I forgot all about it and as a result, could not shoot 4K video until I went back to the store to buy new SD cards. Take note that SDXC cards of at least 64GB size are required, the the memory cards from your last camera might not work. I thought the Sony's were best 'bang for buck' and that I wouldn't have to worry about compatibility issues, and got 2 Sony Class 10 UHS-1/U3 SDXC up to 95MB/s Memory Card in 128GB size.
For adventure or expedition shooting, you probably won't go through as many batteries as say, a wedding shooter. The Camera does come with 2 batteries in the box, and although I've yet to go through more than 2 batteries in a day of shooting, it's worth picking up a 3rd battery as a spare. I stopped using 3rd party batteries and only recommend using original Sony NP-FW50 batteries for consistent performance. Sometimes, there's a deal from Sony to purchase an extra battery when you buy your camera. Be aware that there are fake batteries circulating in the market.
I keep my memory cards and batteries in a MindShift GP2. It's made for Gopro accessories, but will hold 2 SDXC memory cards and 2 Sony NP-FW50 batteries. I number my batteries and cards, and the clear holder allows me to see which card/battery is next up to cycle. It's lightweight and allows me to secure way to store and organize those essentials.
LCD Screen Protector
Before you take the camera out of the box, put an LCD Screen Protector on it. I had mine for just a day before I went back to the store to put a screen protector on, and the screen was already scuffed.
Peak Designs Leash Camera Strap
|The Leash, by Peak Designs
Consider the Peak Design Leash Camera Strap. I frequently take the strap off so it doesn't interfere with me moving the camera around and either snagging (hence ruining the shot) or making some kind of noise that will interfere with the built in camera microphone. I'm used to homemade solutions, but the Leash is the best, lightest and most adjustable design I've seen, and will allow you attach their 'anchors' directly to the camera lugs, thereby eliminating the need for the triangular mounts, which could be a potential source of noise as well. I also bought their Standard Plate, so I could attach the strap to the plate and have the camera over my shoulder hanging lens down, a much more ergonomic position. Note that the strap is thin, and may not be the most comfortable for heavier setups.
|Think Tank Body Bag
Camera Body Pouch/Bag
I like the Newswear Body Pouch that I've been using for years. It fits the a7sii with 24-70mm lens, but with the hood stored in the reversed. I've recently acquired the Think Tank Body Bag, which is larger and can store the same body lens combination (or a larger lens) with the hood in place, ready to use. These bags are the ultimate in flexibility and lightweight for the adventure shooter. I can use them as a chest pouch, mounted as a holster on a belt, or chucked into any backpack. The velcro flap gives quick access to the camera, but dust protection is not so good.
Zeiss Lens Cleaning Wipes and a Chamois
|Zeiss Cleaning Wipes
A friend gave me a box of Zeiss Lens Cleaning Wipes and since then, I've ditched my Lenspen in favor of them. Nothing wrong with the Lenspens, its just that I never throw the old ones away, and I just don't know which have been around long enough to have picked up enough grit on the cleaning pad to be dangerous. The Wipes are one-time-use, so I know they are clean. I bring one wipe for every 3-4 days, plus one spare, or about 3 wipes for week long trip. I also carry a reusable chamois (synthetic, microfiber) to clean, and wipe off fog or moisture from the lens.
|Sea to Summit Dry Bags.
They have nicer colors.
Ultralightweight Dry Bag
The Think Tank Body Bag (above) does come with a waterproof cover, but for extended periods in the wet, and also for unforseen circumstances, I prefer the security of a fully-waterproof dry bag. I use a Sea to Summit Ultra-Sil Nano Dry Sack, which is lightweight and folds small out of the way. The 13 liter size fits my camera with body bag, and a spare lens. Or, a plastic bag stuffed into your camera bag will do in a pinch.