Depends on the terrain and the trip.
If I’m backpacking, it’s all about low volume and low weight, which means synthetic fabrics, which requires more care and longer cables to isolate myself from my mics. If it’s “front country” or a day trip, fleece is a lot quieter. Never cotton, ever, ever. I love those reinforced knee/seat outdoor pants from brands like RailRiders and Fjallraven; I’m on the ocean coast often with lots of sharp bits around. A stretchy neck gaiter like a Buff is a lifesaver for keeping bugs or sand out of your mouth and nose.
I bring duplicates of the most important bits if I can. I bring hand sanitizer, sunscreen, the hiking and backpacking basics. Sunglasses, bat (baseball cap style so it doesn’t interfere with cans). Velcro cable ties so I can hang cables in loops and walk safely. Small three-legged camping stool. One or two types of tiny tripods for the mics. I also bring the included brush with my Rycote fuzzy windjammer, any and all sizes of screwdrivers to service gear in the field, and a bit of gaffer tap. Of course, spare media and batteries. If it’s cold, put the batteries against your body, under your insulating layer, so they won’t discharge as rapidly.
Also wrote this article about going fast and light while field recording: http://sonicfield.org/2010/10/portage-ii-the-ultralight-way/
It depends on where you’ll be going. Some of the things in my bag: Rain poncho (very inexpensive, can take up very little room, depending on how thick the material is), energy bars, water, insect repellant, sun block, medical tape, small flashlight, a little bit of cash, etc.
I also have a waterproof container, which contains things like spare cables, lots of batteries (in small plastic cases), extra SD cards (in a plastic case which absorbs some of the impact if you happen to drop them).
You will have to adjust the contents of your bag according to where you’ll be going and how far I’d have to carry said bag. You might have to add some survival items, depending on where you’ll be going.
Going to a field recording trip is not that much different than another [boring-non-field-recording-trip].
So that means, you should be prepared, wherever you go. If you go into the mountains with wrong shoes, without raincoat, no sunprotection, no food – I’m pretty sure your trip will be a nightmare. Being unprepared could bring you in serious dangerous situations. Here in the swissmountains, snow and ice can come up even in summer and people get lost every year.
So, think about what to bring with, depending also how long and where your trip will be. Also, carrying a backpack full of audio stuff is certainly great for recording, but maybe a bit later in the day, you feel have a sip of hot tee or something to eat.
Also another tip from my side: if you go for a hiking or travel-trip, prepare yourself with maps and information in advanced as good as you can. Google maps would be a help to see what’s around the place you plan to record. If you go to countries with no or only very raw maps, you need to book a local guide.
Maps in digital forms are certainly great too, but if the device gets wet or the power is down, you’re lost. Paper is still a great invention, could also get wet and useless, but you understand what I mean.