Thinking on one's feet is an essential skill in today's fast-paced world where quick decision-making is required. However, if you want to add some variety to your language, there are several synonyms that you can use instead. One such synonym is "quick-witted," which refers to someone who can think and respond quickly. Another synonym is "nimble-minded," which describes someone who is mentally agile and can quickly adapt to changing situations. "Ad hoc" is another term that can be used to describe thinking on one's feet, meaning something done spontaneously or in response to an immediate need. Finally, "resourceful" is a word that describes someone who can come up with creative solutions to problems quickly.