For contractual purposes, my company needed to get several of us Comptia Security+ certified. My travel schedule prevented me from being on the initial list of employees. The others, they gave about three months to study and prepare.
Then, less than a week before the test, I was asked to go ahead and take it as my travel plans had changed. I was travelling home on a 26+ hour overseas trip.
Fortunately, my background and experience helped tremendously. But there were port numbers, encryption details, acronyms, and other items I needed to be able to pull out of the air. I finished reading a Security+ book on the flight, making notes of what I needed to memorize.
I put everything I needed into a memory palace. I not only passed the test, I got the highest score from my company.
That was the most useful thing on the spot. Another useful thing that only involves a small amount of memorization is the Doomsday Algorithm. With it I can give the day of the week for any date, future or present.