Most of these things will come as no surprise to other photographers, I'm sure, but these are the 5 things I simply cannot live without.
1. An L plate. No, not that big red L sticker you put on the car when you're learning to drive! A camera L plate is a metal bracket that screws onto the camera where you would normally attach it to a tripod. These L plates are costly for what they are, and for a long time I dismissed getting one because of that. However, as soon I did eventually try one, I realised how indispensable they really are. Instead of fiddling around with tripod mounts, you simply slide the entire bracket onto the tripod. With some lenses, the weight of the lens will make your camera slowly slide downwards, regardless of how tight you attach it. This L bracket stops that from happening! But the biggest advantage is that you can switch the camera orientation from landscape to portrait in an instant. No having to readjust the tripod. Of course, if you never use a tripod you have no need for an L plate. But if you do, I would highly suggest getting one. Depending on your camera, there are cheaper universal brackets available, so you don't always have to be the Canon/Nikon or whatever your camera brand is. You can save a little on this one if you shop around! Of course it adds a little weight the camera, but that's a small price price to pay. Mine just happens to be bright orange because I decided black was too 'normal', but they are usually black and therefore less conspicuous.
2. A remote shutter release. Of course you can use one of these if you want to get in to the photo yourself, but for me I use it mostly when taking slow shutter photos when the camera is on the tripod. It just avoids any little camera shake from pressing the shutter button. There are different types of shutter remotes, but I use a bluetooth one. You can use corded ones, infrared, or even your smart phone. It all depends on the technology built in to your camera. Whatever type you use, they all do the same job. Some are slightly fancier and allow you to refocus, but it's essentially just the shutter remote that you need.
3. A torch! Why, you ask? Some cameras come with a built in flash, so why not use that. Well mine doesn't, but I do have a separate flash. However I don't always take it out with me, as its big and bulky both to carry and to use. I only take it when I know I'm definitely going to need it. So a very simple, but smaller and lighter torch is nearly always in my bag. It's nothing fancy. I actually bought it for dog walking, because where I live has no street lamps, and the lane I live on, although not busy, has very fast traffic. So I bought it to see and be seen when dog walking at night, especially in winter. But I found it to be useful in my camera bag too, so I bought another one. For me, it has the added advantage of helping out in situations like when I get lost somewhere, or am away from street lamps longer than I thought I would be and I find myself in the pitch black. I'd switched camera bags when I got 'lost' in the sand dunes, and cursed myself for not bringing the torch! So it doubles as both a poor mans flash (woman, in my case), and the obvious, a useful torch!
4. Bin bags! No, I haven't lost the plot. Not quite, anyway! I carry a couple of small bin bags. In winter time, when it's below zero, I put my entire camera, including lens straight into the bag before I get in the car or come inside. I remove as much air as possible, and seal it. That prevents condensation from building up inside the camera or lens from going from sub zero temperatures into a warm room. Once the camera comes back up to room temperature, you can take the bag off. It also doubles for putting anything dirty in it, or even rubbish if there's no bins around. And lastly, it covers my pampering needs, and I can put it on a wet bench if I want to sit! But it's mostly for protecting my camera.
5. Last but not least is my tripod. That's a bit of a cheat item for this list as it doesn't fit in my bag. But it's one of my most useful pieces of equipment. I don't always take it out with me, but if I'm planning on doing any low light or slow shutter shots, it's vital to keep the camera still. The picture is a bit of a funny story, in that my husband took it. He only ever snaps me when I'm completely unaware, and he usually snaps my backside as he thinks it's funny. So here's a rare one, and quite possibly the only one of me you're likely to see taken by him, for that very reason!