I loved my stay at the Diamond Capsule Hotel. It's not really a capsule hotel in the Japanese sense, more a small sleeping area/working area with a magnificent bathroom.
Loved the rainfall shower, the bed (best night's sleep I've had for ages), and a great and reasonably priced breakfast.
The area is pretty far away from Amsterdam proper but there are trams, or you can easily walk it. Amstelven is a really quiet and safe area.