The sauna facility has an attached co-working space. It's paid, but super cheap at just 150 yen per hour (plus a separate admission fee). It comes with full WiFi and power outlets, and the desks and chairs are nice and comfy, just like in a standard co-working spot. Being a new place, both the sauna and the whole building are spotlessly clean and really pleasant. I used the co-working space around 9 PM on a weekday, and I was the only one there. It also seems great for overnight stays.