Beanies
A neoprene beanie is the middle ground between nothing and a full hood. It keeps your head warm without covering your ears and jaw, which suits shoulder-season sessions when the water is cool but not brutal.
Most designs include a chin strap, which matters more than it sounds: a beanie without one will be pulled off by the first decent wipeout. Thicknesses generally run 2mm to 3mm.
Riders who get ice-cream headaches from cold water will feel the difference immediately. For genuinely cold winter sessions, step up to a full hood that covers the ears and seals against your wetsuit collar.