Edoyu (両国湯屋 江戸遊) is a well-known spa (technically a super-sento) in Ryogoku that offers a wide variety of baths and saunas. Plus the extensive facilities includes a restaurant and relaxation lounge so you’ll have no problem killing at least a few hours. Free WiFi is available throughout.
Edoyu (両国湯屋 江戸遊), 1 Chome-5-8 Kamezawa, Sumida, Tokyo 130-0014 « Google Maps »
11:00-09:00 (next day)
Adults ¥2,750 but less than half price for morning visits (6:00-8:00am) and for those who stay less than one hour.