I've done a lot of backpacking routes over the years and there are some items that always make the gear list. No matter where, no matter when, I don't even need to think about them. Here they are:
On a long trek you'll need to re-supply - I carry one of these can openers just in case that's all I can get hold of.
People will tell you when the best time to go to a region is to avoid mosquitoes and midges but nature has a way of surprising you with mosquitoes at 3000m or midges in September. If you get caught without a headnet life can be very miserable.
You definitely need a knife, but not for very much; cutting cord, preparing food such as cheese etc. So I always take a really small and light knife.
I've lost count of the number of repairs I've done with these. From tent pegging loops to rucksack straps. A decent, reusable version is the Nite Ize Gear Tie 12 inch Re-usable Rubber Twist Tie.
I generally take the smallest bottle from the Nalgene Plastic Container Small Travel Kit and fill it with salt - this peps up bland freeze dried meals and will also be useful when you cook your own meals from scratch.
I always include two Shelter System Grip Clips with cord pre-fitted, two Lightwave Dyneema Guylines and two spare pegs. This gives me the wherewithal to add two guy lines to anywhere on the tent which is ideal if the wind is blowing from a particular direction and you want to support the tent on that side. Used on quite a few trips in poor weather.
Even on a bad weather day the sun can get out and attack! I always take sunscreen wherever and whenever I'm going because there will be a time when the sun comes out and if you're walking all day there's often no shelter/protection. I protect my head and some of my face with a hat, but because I use trekking pole my hands can suffer.
These weigh approximately 1.5g per tablet and I'd usually use around one Nuun hydration tablet per day. Adding some flavour to the water encourages me to drink and stay hydrated. A tablet a day is fine for most trips but if it's going to be hot then I take at least two.
On a long trip I'm going to get a shave at some point so I need this mirror. Also, I can check out how bad I look before mixing with normal people again at the end of the trip.
Most mats come with a repair kit these days, if not, you can usually buy one separately and it weighs next to nothing.
This list comes to a total weight of: 189g
Others will have their own lists. A lot of people will take duct tape for example, but I've never needed it. I used to carry a needle and thread, which frankly, given that it must be all of 2-3g I probably should add back into my list.