Battery-operated tea lights. Last year my sister got up several times during the night to use the bathroom, she would turn on the light then open the bathroom door wide and of course light up the cabin disturbing me.
After hearing me complain about it a friend suggested I try these. So when I cruised with my cousin this year I left one on the bathroom counter at bedtime. It gave enough light to light up the bathroom but not disturb me when the door opened. In the morning I would turn it off and stow it on the shelf and it lasted most of the week. (I brought a pkg. of 2).
Sun Princess (2 times), Mississippi Queen, Dawn Princess, Coral Princess (2 times), Regal Princess, Grand Princess (2 times), Diamond Princess, Maasdam, QM2 (15 times)