Toaru Kyuka no Sigoshi-kata (How to Spend a Certain Holiday), is Fuse's special story set between the ninth and 10th light novel volumes.
Tensei Shitara Slime Datta Ken - Toaru Kyuka No Sigoshi-kata Chapter 21
Tensei Shitara Slime Datta Ken - Toaru Kyuka No Sigoshi-kata Chapter 22