Read about Trachtenberg system for mental math. Now choose a random number. Start with 5 digits, with time try to upgrade to longer numbers. Do mental computations. Expected effects: faster and better working memory access, improvement of mathematical tasks. No program or tools required: everything can be done in your head.
Multiply by 2, 5, 11 and 12.
Multiply by a random number between 2 and 12.
Multiply by a random two-digit number. This includes two single-digit multiplications and addition.