There is also a luggage store at Amsterdam Centraal station.

If you aren't up for wandering round town then I would recommend using the Citizen M hotel at Schipol if day rooms are available. It is a modular hotel so the rooms aren't huge but you have more space than Yotel. (http://www.citizenmamsterdamairport.com/). Otherwise there is a Hotel Ibis right beside Centraal station. Both are basic but thats all you want if you just want a shower and to sleep.