Righty ho,
I'm off to the White Room next week for their 'classic singletrack week', pretty excited... What do I need to pack that I'm going to forget? Already sorted tyres and a spare hanger....
Fly repellent (though you might be above the flies with WR). They were nasty out there a couple of weeks ago.
More spare hangers! Brake pads, even more tubes (I only take a couple as I've got good reliable tubeless but others in the group ended up going through loads). More riding kit than you think you need (fresh gloves are very nice). Spokes, if you have 'em. Spare anything that you'll not be able to easily find, in fact. Anything else sensibly portable that you can be bothered to pack...
(I take a spare mech and a whole spare brake, and a bleed kit, since I've got 'em lying around anyway and it's a lot less bother than having to buy one in a hurry. That's probably a bit ott but there's zero drawback.)