Love hotels are a unique Japanese style hotel. I`m sure you can guess what they`re for so I won`t go into too much detai about it. A lot of these hotels charge by the hour, and you aren`t meant to spend the whole night in these places. Love hotels are called "love-ho" in Japanese. A lot of these hotels have some sort of theme, so you can enjoy spending some time in a themed room.
The reason why I think these hotels are interesting because they are uniqely Japanese. You`ll find them everywhere. Aside from love hotels, also be sure to check out capsule hotels (for cheap accomodation), and business hotels (for a reasonable stay).