Karakai Jouzu no Takagi-san: Water Slide

からかい上手の高木さん ウォータースライダー

OVA bundled with the ninth volume of the manga.

Joining Mano and Nakai on their trip to the water park are classmates Nishikata and Takagi. However, before they can decide on what to do first, Nakai runs off with Nishikata, leaving his saddened girlfriend behind with Takagi. Mano explains that Nakai promised to ride down the water slide with her—which is rumored to make tandem riders fall in love. Unsurprisingly, Takagi devises a plan to regroup the couple in a way that also permits her to spend time teasing Nishikata.

(Source: MAL Rewrite)