There's a sex swing in one of the rooms at a SM love hotel in Kanagawa, south of Yokohama.
Here:
http://www.hotenavi.com/aiaihinkan/index.html
Room 207 has the sex swing, and some other SM equipment, and there are sex chairs in other rooms.
You can book a room 30 minutes in advance ... Which gives you a slightly better chance of getting the room you want.
The rates are fairly good, and they have nice long "service times" which are calculated from when you check in.