Akihabara Bay Hotel
This is a capsule hotel in the strict sense: stacked sleeping pods with a curtain, a light, a power socket and not much else. It takes women guests only.
Akihabara Station is a few minutes away on foot. Lockers hold luggage, a shared lounge and bathrooms do the rest. Capsules are cleared in the middle of the day for cleaning, so plan to be out. Simple, safe, well run.