What will the Hole-Digging Club do during summer vacation...?

🥚 Tests over! Did terribly!
🥚 Summer break!
🥚 It's hot!!
🍄 You're so loud...

🥚 Hey, Shimeji-chan, will you be showing up for club?
🥚 Doesn't seem like you have to go, though...

🍄 I'm thinking of showing up every day, actually.

🍺 I'll be coming every day, so you can come whenever you feel like it

🥚 Huh. Wasn't expecting you to be so motivated.

🍄 Well, my place's jam-packed with all these weird machines right now and I'm pretty much confined to my closet.
🥚 How do you live like that...?
🍄 Somehow.

There is a suspicious apparatus shown on the last frame. What does it do and what is Sis planning.