I've bought my teen boy some of the Hawkins Bazaar books over the past couple of years (he's 15)
There was an optical illusions one and "F-in Exams" some of the boo-boos written in exams
Lynx
lots of special offers at the moment
I buy a 3 pack of M&S trunks that aren't black (he wears black for school)
Microsoft Points
Pre-loaded Costa/Starbucks card
Art pencils (I bought a tin of 2H through to 2B pencils from The Works)
Money tin (The Range has them for £1. You need a tin opener for them)
Coin holder (I picked one up at a garage, it holds 5 x £1 coins to keep for an emergency. No that doesn't mean an Emergency Bottle of Fanta, DS)
Don't know the price so i might be a 'present' present rather than stoking gift, but my DD got a charging phone case (I'm not sure what they're called , but she charges the case and it charges the phone )
Memory stick