Depends on the lighting you have in there. And, do you want something that trails or stands upright? Large or table-top sized?

With sufficient lighting, you'll likely find plants thrive in a bathroom setting.

In dry areas, the extra humidity of the bathroom is a good thing for many plants, especially ferns and other rain forest species.

The only thing I can think of is snake plants. They grow straight up and need no light. They are inexpensive and easy to maintain

Here's a bathroom with plants.

don't forget orchids