This is always a difficult question that I face every year! Sometimes there is a big item they need like a bike or a drawing tablet - but often there isn't anything obvious.

Something that I'll often do is get them digital goods, because they take up no physical space. Often, it winds up being a new downloadable video game or an expansion to a game they already own. I try to be thoughtful about what they like & I research the games in advance to find something unique to them and fits their interests. I download the game to a device in secret on Christmas Eve and then find something physical that they can unwrap (like cardboard with a game picture inside or something). Then as soon as they open it I explain why I choose it & why I think they'd like it & etc and I get in their hands quickly so it feels more visceral.

Others will disagree about this strategy, I know, but it prevents accumulating a lot of junky unnecessary items. My kids are in middle school & high school now & they have very few possessions compared to their friends. What's nice is that they take a sort of pride in only having few high quality items rather than a room full of junk.

We also stop buying them (& ourselves) "necessity"-type items like clothing beginning around September, and then they get to unwrap a bunch of boring things like socks & pants & and soap at Christmas. Lol! Are we the worst? Maybe.