There are several synonyms for the phrase 'in good form' that can be used to describe someone who is physically fit. Some of these include 'in good shape', 'in good health', 'in fine fettle', 'fit as a fiddle', 'hale and hearty', and 'full of vim and vigor'. These expressions can also be used to describe someone who is mentally sound and performing well, such as 'on top of their game', 'sharp as a tack', 'quick-witted', and 'mentally agile'. Overall, there are numerous ways to describe someone who is in a good state physically or mentally, each with their own unique flavor and nuances.