Headwork is a term that typically refers to mental activity, such as thinking or problem-solving. If you're trying to open up your vocabulary and mix up the terms you use to describe intellectual pursuits, there are a variety of synonyms to consider. For example, you could use the phrase "cerebral exercise" to describe mental work. Other options include "intellectual labor," "brain work," "mental effort," and "cognitive engagement." You could also use phrases like "mental gymnastics," "intellectual challenges," or "cognitive exertion" if you're looking for a more playful or creative way to talk about your intellectual pursuits. Ultimately, the right synonym will depend on the context and the tone you're aiming for.