Here are my go-tos, but bear in mind our family is uh, unconventional and tends to joke a lot so some of ours are a bit off the wall (but might spark an idea? I'm not sure):
-Trading cards. For ours, it's Match Attax soccer cards, but ymmv.
-Tee shirts for his favorite bands. Target sales, Hot Topic sales, thrift shops, etc are my friend for this.
-Small LEGO sets he's been wanting (he's still in that in-between where this appeals to him, but older teens may not still enjoy, depends)
-Books
-Deodorant. Literally. So. Much. Deodorant.
-Bath bombs and face masks - he's v. secure in his masculinity and loves these lol
-Variations on Rubik's cubes. We've done 3x3, 4x4, 2x2, the new one with the disappearing heat-sensitive colors, etc.
-Pride stuff. Anything with the bi flag on it is always appreciated, and makes him feel loved and seen, so I tend to slip extra stuff in with that when I can.
-Various fidget toys. Right now he's obsessed with those 3D printed slugs.
-Multi-tools (those little snowflake tools that are a screwdriver and wrench and things like that in one are cool, or a hammer with multiple other tools in the handle)
-Batteries. He uses a ton of AAs for his gaming controller and headset, and while I'm more than happy to provide them whenever needed, slipping a pack into his stocking also helps fill it out a little. He also has some very specialty batteries for a little robot he has, so I always put these in? So if your kiddo has any like... specialty equipment, chargers he always goes through that tend to get worn, etc, these can be a good one.
-For a while mine had a habit of losing the remote to his TV in his room, so I found a deal on like, a 5-pack of replacements... it's a good jokey kind of thing to slip in. Anything that he loses often or goes through a LOT of, I tend to slip in.
