Abaci

An abacus is about as simple as it gets. But only "about": loose pebbles in slots engraved in a wood or stone countertop preceded abaci. And there were "counting rods" before that.

It's fun to learn to use an abacus, and challenging to get good at it. It requires both mental and physical agility. I know Japanese students used to be trained and tested on their speed. There were even national contests. I've read a highly skilled abacus user could outperform an unskilled calculator user. I'm not sure whether that says much about abaci and calculators, but you've still gotta be proud of the abacus user.

Suan phan

This is a Chinese abacus, with 2 beads above (in "heaven") and 5 below (on "earth") the horizontal bar. The extra beads can help make operation faster as numbers can be represented in two form.

This is actually a desk ornament, measuring about 4" across. It's in brass, mounted on stone.

Soroban

This is a Japanese abacus with 1 bead above and 4 below. I think the reason for so many columns is so that the operator can do multiple calculations on different parts of the abacus.