The Slime Diaries: That Time I Got Reincarnated as a Slime

Tensura Nikki Tensei Shitara Slime Datta Ken; The Slime Diaries: That Time I Got Reincarnated as a Slime; Tensura Nikki: Tensei Shitara Slime Datta Ken; 転スラ日記転生したらスライムだった件

This talks about Rimuru and his friends spending their free time doing something fun together. A spin-off from the main series "That Time I Got Reincarnated as a Slime"