Kanji / Hànzì (漢字 / 汉字) are one of the harder parts about learning Japanese or Chinese. But there are two pairs of characters that are, simply put, awesome.
凸凹 (deko-boko) — first seen in Japanese, means “ridged” or “rugged”. (I call them “Tetris pieces”, even though the latter isn’t a valid Tetris piece.)
乒乓 (pīng-pāng) — seen and used in Chinese 1B, means “ping-pong” or “table tennis” (if it had/has another meaning, I’d be interested to know).