Add a splash of paint... with the landlord's permission of course
They may even cover the costs if it's in need of a coat or two! Either a nice bright white or a pop of colour on the walls and door can make a big difference to a drab bathroom. If you can't tackle the walls, add some colour to bathroom cabinets and vanity.
Here we're talking door and cabinet knobs and handles, taps and faucets, and even light fixtures. It's quite easy to do and will make a big difference in lifting the room. Be sure to store the old ones away so you can switch them back when you move out.
If you can, replace the mirror. Most rental bathrooms will come with a stock standard one fixed to the wall, so why not replace it and add a little character with a statement mirror that reflects your style. Look around at garage sales or op shops for pieces you can upcycle.
Replace old or mouldy sealants
If your silicone sealants or your caulking are coming loose or going mouldy, replacing them is a really easy, low-cost job you can tackle yourself.
Inject colour, style and personality
Just because it's a bathroom, doesn't mean you can't add your favourite artworks to the walls. Even some nice bright towels, face washers, bathmats, baskets, accessories or a funky shower curtain, will lift a dreary bathroom.
Finally, use a little elbow grease
Often it makes a difference just to do a really good deep-clean with some heavy duty cleaners and scrubbing brush... tiles, grout, soap holders, in and around taps and faucets, behind toilets and under the cistern. You'll be surprised at what a difference it will make.