It's difficult to really put your own decor personality on an apartment, especially if you can't paint.  Apartments generally are made up of small rooms, so you are challenged on how to really make them stand out without investing a fortune into a place that you will eventually be moving out of. 

With small rooms, you need to make your accessories work together.  A tropical theme would work well in a small bathroom.  As you cannot paint, focus on your towels, shower curtain, floor, and wall art. 

Fluffy towels in moss green, deep brown, and cream add a luxurious touch to your room.  A shower curtain with a palm tree may be over the top, but a neutral curtain with some interesting hooks would be fitting in a small bathroom. 

 Add some visual interest to your floor by choosing a seagrass area rug such as  the Anji mountain seagrass rug with brown border.  It's muted colors will fit right into a small room.

Create your own wall art with pressed palm tree leaves.  Place your leaves inside a matted frame and hang on your wall.  It's inexpensive wall art that you can create in no time at all.

Don't be deterred by your inability to paint.  You can put your own stamp on your small bathroom in just one day and for much less money than you may think.  Have fun!