That's a pretty good suggestion from LapLap -
IMHO the LaQua spa would be a fun experience if you have an 8 hour stopover during the day-time. However, if we're talking about an overnight stopover... I would rather not use that facility for an overnight stay. I can't get to sleep in a huge hall of strangers who are watching TV on reclining chairs.
Note - The LaQua spa is not quite as easy to reach as LapLap's post suggests. I think this is the best route:
From the Keisei Ueno station, you should walk South for a couple of blocks until you reach the Ueno Okachimachi subway station (for the Oedo line). From there, you can catch the Oedo line to Kasuga.
The LaQua spa is directly next to Kasuga station. (*Basically it's the same location as Korakuen station).