Cold plunge accessories divide neatly into things that make the session possible and things that make it look good in a photograph. Three of them genuinely change how often you use the tub, and the first of those is the one people skip because it is dull.
The cover
The dull one, and the most important. An insulated cover holds the cold overnight, keeps leaves, dust and sunlight out, and cuts the ice bill or the chiller's run time more than anything else you can buy. It also stops a full tub being an open hazard in a yard. If you buy one accessory, buy this.
A thermometer you can read
Floating or wall mounted, cheap, and the thing that turns a habit into a practice. You cannot judge water temperature by feel within five degrees, and the difference between fifty and fifty five changes how long you should stay in. Most chillers display a set point, which is not the same as the water temperature at your shoulders.
Booties, gloves and a step
Hands and feet hurt first and hurt most, and for many people they are what ends a session before the rest of the body has had the exposure. Neoprene booties and gloves are the accessory that lets people go colder or stay longer, and they are not cheating. A step and a grab point beside the tub matter more the deeper it is.
A mat, and what to skip
A non slip mat beside the tub is worth it: getting out cold and clumsy onto a wet deck is the riskiest moment of the session. What to skip, at least at first, is anything decorative, and any accessory promising a physiological effect. The tub is cold water; nothing you hang on it changes what that does.
Questions people ask about cold plunge accessories
What is the one accessory worth buying?
An insulated cover. It holds the cold, keeps debris out and cuts the ice or chiller run time more than anything else.
Are booties and gloves cheating?
No. Hands and feet hurt first and often end a session before the rest of you has had the exposure. They let most people go colder or stay longer.
Do I need a thermometer if the chiller has a display?
Useful, yes. A set point is not the same as the water temperature at your shoulders, and a few degrees changes how long you should stay in.