I'd ditch the multitool and the thermos and the booze.
Typically I consume more snacks than expected, so take plenty.
Better take a spork so you don't need to eat your Wayfarers with your fingers or the multitool.
I'd take sunscreen, toothbrush, toothpaste, deodorant, loo paper, trowel, pencil stub, soap. These can be just a few grams each.
Take 2 or 3 spare poly bags. They are very light and tend to come in useful, if only for carrying out trash.
All all costs, keep your sleeping bag dry. So take a suitable drybag! You probably want to keep your spare clothes dry too. Don't rely on your rucksack to be waterproof.
In summer don't forget the insect repellant; you can also take a light pair of shorts for swimming.
If you are with a companion, some things you don't both need to take; this also applies to some spares.
I like to have a checklist as it helps me to pack quicker without forgetting stuff.
Have fun!