Mine are 16 and 17 we've more or less gone down the edible route and I don't worry about the stuff being similar for them both.
Last year I put in big bags of crips and popcorn, bags of sweets, jars of nutella, hot sauce, new drinks bottles and mugs, new head torches, new penknife for one, usb sticks, smellies, magazine, book, headphones, socks, gloves and guitar picks.
So far this year I've got DS1 a 3d pen in the sale, a thing for strengthening your hands for DS2 (for his guitar playing). I'll stick to the food.drink items and more headphones and a magazine/book. Thinking some kind of fidget toy for Ds1, a shower timer for DS2, some beard stuff for DS2 and maybe a shaving subscription for DS1. Also a nice nail clipping type set.
Other things we've had in the recent past: mobile phone covers, wallet tools, paracord stuff, emergency whistles and bags, sporks (for DofE). Stress balls, tins of strong mints, chilli chocolate, stationary type stuff, wallets, pants/socks etc.