If by improve you mean decorate, then stencils can be cool like this https://www.hometalk.com/2670067/stenciled-ceiling-medallion-in-festive-metallic-paints
What exactly is the issue? Is there water damage? Stains? Paint is the easiest solution.
What is there now?
Deborah...Sure you can....just painting will make a big difference.
But if you have any stains from water damage fix that before painting.
Do you want a type of decorating technique for the ceiling or is there some type of damage?
If you are just wanting something different in the bathroom, you can always go with paint like others have stated above, but if it isn't too large, you could plank wood over the current bathroom ceiling to give it a upgraded look! I think it's gorgeous when people use cedar or other wood! Here's a couple of examples!
Use joint compound, stamp designs ,paint and seal. Kinda stucco look, designs are as many as your imagination
You can paint the ceiling a cool color or add wallpaper to the ceiling. They have removable wall paper that would you could use for a not so permanent look.