Cases and gig bags: the guitar leaves the house or it does not
A gig bag is padding and a shoulder. It protects against scuffs, weather and being leaned on. A hard case is a shell, and it protects against being dropped, sat on, or stacked under other luggage. Neither is better; they answer different questions. Gig bag vs hard case works through the decision, and the cases roundup has the picks.
Capos: the mechanism is the whole product
Every capo clamps a bar across the strings. What differs is how the pressure is applied. Fixed-tension spring capos — Kyser, Dunlop Trigger — are instant and one-handed, and their fixed pressure pulls some guitars slightly sharp. Adjustable capos — Shubb — take a moment to set and then hold tuning better. G7th's Performance 3 splits the difference with a pad that mechanically follows the fretboard radius. The full argument is in the capo roundup.
Picks: buy a variety pack before you buy a favorite
A .50 mm pick flexes and smooths out a strum. A 1.14 mm pick barely moves and puts every bit of your hand's motion into the string. Which suits you is a physical fact about your hand, and no article can determine it. Dunlop publishes the thickness of every Tortex pick in millimeters and color-codes them by gauge, which makes the experiment trivially cheap. Details in the pick thickness guide.
Straps and strap locks: buy them together
A strap costs very little and the specification that matters is the adjustment range — Ernie Ball publishes 41" to 72", which covers essentially everyone. The more important purchase is the strap lock. The single most common way a beginner guitar gets damaged is a strap end slipping off a strap button while the player is standing. See strap locks.
Stands and hangers: the accessory that increases practice
This is the one on the list with a behavioral effect. A guitar you can pick up in a single movement gets played several times a day; a guitar in a case in a cupboard gets played on Sundays. A floor stand is cheap and portable and can be knocked over. A wall hanger cannot be knocked over and needs a stud and a drill. Wall hanger vs stand covers the trade properly.
Cables, amps and metronomes
An electric guitar needs a cable and something to plug into. A headphone amp costs a fraction of a combo and lets you practice at any hour, which for most beginners is worth more than volume. A practice amp is the answer when you want to be heard in a room. And a metronome is the accessory that most improves your playing and least improves your enjoyment — buy it when you are ready to be told the truth about your timing.