Seki attempts a game of 'Topple the Flag' on his desk, going to extreme lengths to scoop as much sand as possible from a pile without toppling the flag atop of it. Later, Seki brings a pair of cats to school, prompting some jealousy from Rumi as he keeps their cuteness to himself.

[Source: Wikipedia]

During class, a schoolboy named Seki-kun is always creating things to entertain himself. All he cares for is creating things with an artist’s level of perfectionism and a craftsman’s attention to detail. Real dominos, shogi with original rules, a flip book with sounds… Even Yokoi-san, who sits next to Seki-kun, always gets sucked into watching his games!

[Source: HIDIVE]

Two unaired episodes included in the DVD Set release of Tonari no Seki-kun series.

#1. Field Trip (Rinkan Gakkou) (first volume)
#2. Snow Playing (Yuki Asobi) (second volume)