If you want to go to the toilet, you can say:
A.
Excuse me, I have to go to the men’s/lady’s room, go to the rest room, to wash one’s hands;
B.
Excuse me, I have to relieve oneself or to see a man about a horse, or because nature calls.
C.
Excuse me, where can I powder my nose, or where to freshen up?
D.
All of the above are ok.