I carry a powerbank and various USB C adapters so if someone’s device dies I can help them out. It is a lot cheaper and smaller than carrying around seven different cables.
On top of that I carry Tylenol and bandages.
I used to carry a full first aid bag but kept forgetting to restock on supplies.
Aspirin for random headaches, a small sunscreen bottle to reapply throughout the day (I live in Ecuador at 3k altitude, skin cancer is a real concern), a charger with a usb c cable, a micro b cable, a mini b cable and a lightning cable cause you never know. A Bluetooth gamepad for my phone to play gamecube games when in have some time to kill. Scotch tape, silicone glue. Usb c otg, micro usb otg. An external 500gb hard drive, a waterproof poncho because I alwayd misplace umbrellas. A powebank, 2l bottle of water for the whole day. Those are some of the things I always carry on my backpack