my 18yr old has mainly sweets in his, he doesn't even open the jokey type things that get put in, and pens/pencil etc normally end up finding them in the stocking the next year when i pack it up again.
The wallet ninja looks good.
Cheap ear phones, ds2(13) has those beats ones, but they are terrible and keep breaking so he likes the cheapy ones that come with the iphone or from poundland as at least they can be thrown away without guilt when they break.
starbucks/costa/subway £5 gift cards, good for them to have in wallet for emergencies.
otherwise its slightly nicer deodorants and shower gels than they normally use, e.g they use lynx every day so i buy ted baker or fcuk in boots, usually on a3 for 2, or lynx do mini tins with mini products in great for stuffing in gym bags.
fluffy slipper socks, used to buy posh ones from joules/superdry etc but they prefer the 2 packs form Tesco think its 2 pairs for £4 and i split them between the two boys.
sweetwise i will buy chocolate coins, freddo frogs, jelly beans, you can usually find lots of small Christmassy themed chocs and sweets in the supermarkets.