Vacuum roll bag: I don't agree with the Which test, I've used a vacuum roll bag for clothes for years and it vastly reduces the amount of space clothes take up. Yes, it creates a large rectangular brick but I pack round it. Roll clothes up so they crease less.
A capsule wardrobe: it sounds wanky but a small amount of clothes that all go together cuts down on the packing space.
Teva sandals. My god they're comfortable and I've walked up a mountain in mine!
As many things as non-liquids as possible: baby wipes rather than cleanser, laundry powder/leaves rather than liquid (I pack light and hand wash if I need to), stick deodorant etc. Also, I've often done what PPs have suggested which is to order from Boots at the airport - make sure it's AFTER security and if you're transiting get them to put your stuff in a sealed security bag. Some UK airports have ditched the 100ml rule now but some haven't, never mind ones abroad. If you want to bring remaining liquid stuff back with you, take some empty travel bottles.
Noise cancelling headphones and eye mask. Not just useful for the flight but noisy hotel rooms too or ones that have too much light. Background noise downloaded on to a phone/iPad is also useful!
Microfibre travel towel.
Multiway charger. Check the adapter you need and take a spare. Power banks are also helpful.
Vicks First Defence spray. Yes, it's like sluicing your sinuses with battery acid but it's good for fending off all the bugs you'll have breathed in on a long flight.
A door wedge or one of those safety door locks that prevent your door being unlocked from the outside if you're staying on your own.
AirTags - invaluable if your bag goes missing and if you're travelling on public transport you can make sure no-one 'accidentally' walks off with your luggage.
A packing list, cut down to the minimum. Double up on stuff where you can (shampoo also doubles as shower gel etc) and don't take anything you don't actually need, while making sure you've got the stuff you do.