There's a "Spa Hotel Nagomi" near Kamata station:
http://www.spa-nagomi.com/

A tranquil retreat it ain't. Expect something quite low rent. But it's convenient to the airport, it has hot spring baths, and it has proper hotel rooms. Most of the sento in the area will close overnight and those that stay open will often offer only the most basic of accommodations - perhaps a reclining seat or perhaps a capsule.

Other people have mentioned the Oedo Onsen Monogatari as an option for an overnight stay. I don't really see the attraction, but YMMV.