Hmmm, it is close. "Nani o shimasu ka" comes off to me as what are you going to do. However, I'm sure if you ask someone that and they are not doing anything it would be a polite way of asking if they would like to do something. I'm not advanced enough myself to know the direct translation of what do you want to do. I believe it is something like "Nani o shitai ka?" That is a guess though.
As for the bathroom, I'm fairly sure it is "Otearai ni doko desu ka?