SEO hidden words to help searchability?

One of my client's websites has a list of spa services that don't use the traditional words like 'pedicure' or 'manicure' to name their treatments, but they find a lot of people search those terms and they want to direct them to the correct pages. Is there a way to have this copy hidden on those pages so that search engines find it, or is that something that google would penalize them for? Or do you have other suggestions?