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/