The small stuff matters more than people expect. One worn screen, one missing mouthpiece, or one annoying charger problem can make a good device feel worse fast.
Quick answer
Keep extra screens, a backup mouthpiece or mouthpiece cover, one seal or gasket set if your device supports it, and one official wear-and-tear kit if the brand sells one. Add capsules only if you already use them. Add a backup charger only if losing yours would immediately mess up your routine.
Screens: easiest spare to justify
If you only keep one type of replacement part on hand, make it screens. They are cheap, easy to lose, and one of the first parts to make a device feel worse.
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Mouthpieces: worth backing up
If a mouthpiece gets worn or lost, the whole device starts feeling more beat-up than it really is. One backup is usually enough.
Seals and gaskets: buy them before you need them
These are boring, but they are exactly the kind of parts you notice once they start going bad. One set is smarter than scrambling later.
Wear-and-tear kits: usually the smart buy
If the manufacturer already bundled the common aging parts together, that is usually the easiest answer. It is one of the few times the pre-packed option is often the right option.
Bottom line
Keep screens, one mouthpiece-related spare, one seal or gasket set if your device supports it, and one official wear-and-tear kit if your brand makes one. Add more only if it clearly fits your routine.