Since they're usually white, towels, rugs, and shower curtain are always a great way to add color.
You could hang art or photos by attaching lightweight frames to the walls using 3M adhesive strips, and you can also add some life with a houseplant or two.
Add lighting and mirrors to expand the space.
Command adhesive products are great for rentals because they can be removed without damaging the walls.
Stick with small accent pieces rather than larger ones that can over power the space.
Accent with bright colors to also lighten the area so that it appears larger.
Check out removable wallpaper !!! There are so many styles from traditional plain or printed to fun funky patterns.
You could add some fun art work or signs!
Give tile a power wash and check into temporary tile coloring. Many companies do both procedures for under 100.00 dollars. Then repaint walls and trim, ceiling if needed. This can be accomplished with a sample sized containor of ceiling and trim paints. Samples are inexpensive. Look for paint sales for the main color used on the walls - a gallon will do two coats on most bathrooms. After you clean and brighten the space, choose an accent color which you love. The Dollar Store is great for shower curtain liners, plastic organizers, lots of things which would cost you much more at a Walmart, etc. Splurge on a great shower curtain... think of it like a beautiful painting for your bathroom. Baskets are beautiful naturally colored, or spray painted to reflect your decor . A removable cabinet to hide away bathroom clutter works well to keep your bathroom sleek and easy to clean. You can easily take it with you if you move. I love having a living plan in the bathroom. Philodendrons will live happily in low to not much light, require no care and benefit from the moist environment of the room. It’s the only plant I can grow, no green thumb necessary and they flourish and grow beautifully. Good luck!