As a kid, I liked arithmetic function machines that "magically" force a particular output or allow one to quickly deduce the input from the output. These are usually based on non-obvious (to a kid) identities. I would invent my own and then ask family members to play along. I'm not sure if this sort of puzzle game has a name?

A simple example:

- Multiply your number by 2

- Add 10

- Divide by 2

- Subtract the original number