Let it be a small keychain pocket toy, something like a fidget cube. Let's make it play some sort of small games, like tic-tac-toe or puzzles. Its body should have a 3x3 button grid with two color backlit on each button.
I decided to use a 3D printed body, a two-sided PCB, clicky mechanical tactile switches and a cell battery CR2335 a very small microcontroller and no additional chips, like shift registers. And this left me no other ways to conserve pins than to organize LEDs and buttons in matrices.