Moving and Renter Kit
The unglamorous box of things that make moving into and out of a flat less expensive: felt pads and floor protection, anti-tip straps, spare fittings for flat-pack, levelling feet, a no-drill hanging kit, and a wall repair and making-good set.
The wall repair kit is the one worth buying before you need it. Unfilled holes are what deposit deductions are actually made of — not the drilling itself — and filler needs a second pass and a day to dry, so it's a job for the week before the final inspection rather than the morning of it.
The anti-tip straps are the other one. Four small holes, and they're the difference between a tall chest of drawers being safe around a toddler and not.