My Neighbor Seki and Rumi's Phenomenon.

The boy who sits next to Yokoi in class, Seki-kun, is always up to something at his desk. He manages to somehow play very elaborate games without attracting the teacher's attention. From dominos, to shogi, to go, to mecha models, to playing with cats and more, he always has something new to do. Yokoi often finds herself getting reluctantly interested in his games, even though they always seem to end up getting HER in trouble with the teacher!