The following are appropriate in any situation:

下面是任何场合都恰当的用法：

Bathroom

–Example: I'm gonna [going to] go to the bathroom.

例子：我要上卫生间。

Restroom

–Example: Where is the restroom?

例子：洗手间在哪儿？

Little boy's/girl's room

–Example: I'll be right back, I need to use the little girl's room.

例子：我马上回来，我要用下洗手间。

Powder my nose

–Example: Excuse me, I need to powder my nose.

例子：不好意思，我要去趟盥洗室。