Player-Facing Content
Pretty much every sourcebook has content that would be useful to players, but it's not all implicitly player-selectable. When I refer to magic items below, I'm talking about special unique ones, not DMG items with a splash of extra flavor.
Lost Mine of Phandelver, Rise of Tiamat, Storm King's Thunder, Tales from the Yawning Portal, and Waterdeep Dragon Heist have magic items.
Hoard of the Dragon Queen and Out of the Abyss have magic items, plus alternative Background features. They're not full backgrounds (skill picks, equipment packages, etc) just the "special ability" part.
Princes of the Apocalypse has new races (genasi) and spells, but both are available (along with other things) in a free online download, the Elemental Evil Player's Companion. The spells are reprinted in Xanathar's Guide to Everything as well.
Curse of Strahd has one completely new background, plus magic items.
Tomb of Annihilation has two new backgrounds, plus a few magic items.
Waterdeep: Dungeon of the Mad Mage is the notable exception. It doesn't have anything truly player-facing: no unique magic items and no character options.
DM-Facing Content
All of the above sourcebooks have monster entries. Some of those monsters are actually named NPCs, which may not make them terribly useful outside the context of the adventure.
By the same token, none of them really have any special unique mechanical aspects that would make them a worthwhile purchase if you're not actually going to run the content. They are certainly full of ideas you could steal (like specific Chase Complication tables for the Chase rules in the DMG), but nothing groundbreaking - nothing that is super-must-have-your-game-is-suffering-for-lack-of-this.
If you're looking from a purely money-on-the-barrel kind of angle, you would be far better served with books like the Sword Coast Adventurer's Guide and Mordenkainen's Tome of Foes than any of the adventure books.
For a deep dive on setting lore (rather than mechanics), you actually might even be better served by looking further back. Depending on where you're playing, the 3.5 Forgotten Realms Campaign Setting is probably one of the most dense setting books, and the Ravenloft one (published by White Wolf under their Arthaus imprint) is equally fantastic.