My rental bathroom came with two main problems: it's tiny (about 4.5" x 5") and has basically no storage. At first, I painted the room a pale gray (that read as lavender under the unflattering CFLs), put in a cheap over-the-toilet étagère, and hung a few prints by a favorite artist. Cut to a year later, and I still wasn't crazy about the color or storage solution.
I had chosen the light gray-ish color because I wanted to avoid having to repaint whenever I move out, but it always felt too wishy-washy. I realized I would be happier with a stronger, deeper color. Lesson learned: don't compromise on paint. Either keep it simple (white) or go with your gut.
The paint I chose was a different, overly bright green from Valspar called "tranquility" or "meditation" or something similarly deceptive. Once it was on the walls, my fiancé hated it, and I wasn't happy either. Not wanting to waste the second half of a $30-something gallon, I mixed in leftover blue and black paint and bought a $5 metal stirrer from the hardware store downstairs. If you're going to mix your own paint color, my advice is to go slowly.
To deal with my storage problem and get rid of the Walmart over-the-toilet unit, I started by paring down to the essentials. I moved bottles of cold medicine, etc., to a designated container in my coat closet. Things such as extra toilet paper and my hair dryer now reside under my pedestal sink. To conceal them, I sewed a skirt with leftover striped cotton fabric; the velcro on the fabric side is sewn, and I used adhesive velcro for the sink side. (If I have any problem removing it when I move, I know I can use a product like Goo Gone.)
Other projects included adding an Umbra toilet paper holder and coordinating towel ring, a small vintage crate, and an acrylic shelf; buying a cork bath mat; and sewing a custom stall-size shower curtain. I also turned a spice jar into a container for matches (using this trick from Chez Larsson to hide the striking surface) and brought in a large print that had been hanging over my bed.
Altogether, I spent about $120. While I ended up putting more effort into the bathroom than I anticipated, I am really happy with the look and functionality of the result.