Those looking for a "brain gym" might consider practicing arithmetic with an abacus.
The abacus, also called a counting frame, is a calculating tool used to perform arithmetic processes. Abaci were in use centuries before the adoption of written numeral systems and are still used by merchants in Asia, Africa, and other places.
The user of an abacus is called an abacist. I keep an abacus in my study, which I use on occasion. I guess that makes me an abacist...