If you’re using a larger camera using a shoulder sling rather than a neck strap. Makes an enormous difference.
I prefer the Hama ‘Quick Shoot’ because the mounting plate is flat so you can still rest the camera on the base, and it also has a secondary tripod mount screw hole, so you can attach a baseplate. There are lots of different shoulder slings to choose from though.
If you have a large lens on the camera, getting a hip holster to go with the shoulder strap. Another massive difference as it puts the majority of the weight on your waist instead of your shoulder.
I use the Hakuba camera holster GW-PRO G3 - Japanese link to the company itself, but you an find them on Amazon and other places. Again, there are many models to choose from, this is just the one I like.
Cotton bandanas, extras of them too. I generally have 2-4 with me. They are good for sweat, cooling off if you’re too hot (get them wet), as pads to rest your arm or camera on if you have jagged rocks or something similar that you’re trying to brace against, for making quick pouches to carry things, etc, etc, etc.
A larger hand lens. Not one of the tiny ones that are common with botanists and geologists, but a 4-5 cm folding one. Makes peering at small things much more comfortable and it’s large enough that you can take photo through it. Also, in an emergency, in sunny conditions, it’s big enough to use for making a fire and things like that.
I use a pocket magnifier kind of like this one
A small, high powered flashlight. one of the ones that can be ramped up to be extremely bright for a few hours run time (eg 800-1200+ lumens), or turned way down for tens of hours run time at the low setting (eg 10-15 lumens). For nighttime and cave photography I use one of these now instead of a flash. Hold it in the same hand that does the focus so it’s up near the end of the lens. At the bright setting you an get relatively long shots, and at the lower settings you can get closer shots without washing them out. Some of the small torches like this have tripod mount screws so you can even mount them directly to the camera flash shoe if you want.
I have a few of these. The E200s Angel Eyes is a good choice, but the dual lights can sometimes make for odd shadows when photographing things up close, as in this Glyphodes bivitralis moth observation. This has a tripod mount, which is really nice, as well as a magnetic back so it can stick to things. I also use several of the Fenix torches and headlaps which are excellent quality.