1) Shower curtain swap. Many showers have obscure glass doors with aging chrome trim. Others have curtains hanging several feet from the ceiling and not quite touching the floor. A new curtain, hanging from the ceiling and brushing the floor, will add color, softness, hide the unattractive door, and look great. To make a longer curtain, try looking for a twin sheet at a local discount store, or purchase a curtain and a coordinating pattern to make a bottom (or top) border. Use a tension rod in the case of glass door showers, and hang a new rod higher in other showers.
2) Store it. Removing clutter will make a huge difference in the space. No cabinet space? Hang shelves over the toilet and even over the sink(s). Use the space inside your wall. Canisters (ceramic or glass) hold tissues, cotton, and small items and look nice. Outfit the interior of your cabinets with slide out drawers and even towel bars to maximize storage. Pedestal sinks are more challenging and if a Velcro attached skirt (does not need to be ruffled and fussy) doesn't achieve the right look, try placing a large wicker hamper underneath to store extra toiletries, tissue, paper, etc.. Often, a small chest or table with drawer storage can fit in a bath. Not only does it add storage, it adds character and interest.
3) Freshen up the floor. A simple area rug...not a bathmat... can work wonders. A nylon indoor-outdoor rug, or even a nylon/polyester throw rug can make the space look new. Can you clean the grout in the tile? If not, try re-coloring to a darker shade that works with your new shower curtain. Carpet squares are an option to cover a really bad floor - especially in a rental - and can be removed or replaced easily.
4) Paint, paint and more paint. One of the simplest ways to change any space is with color. In small baths, I don't advocate using many bold accessories since it adds clutter. A new wall color, wall treatment or even trim color can make the bathroom look completely different. How? Add horizontal stripes to make a room look longer, vertical ones to add height. New trim color, a richer wall color, and even a faux effect are weekend projects.
5) Light it up. Often, just switching the old fixture with a new one makes a huge difference; even new globes can infuse more light. How about swapping your overhead bar for a larger one? A five light fixture will give more light than the two-or three light fixture that is there. A bit more work is to replace a bar fixture with wall sconces. You will appreciate the improvement in light. Hang a small pendant near the toilet for ambient light. Even adding a small table lamp on a safe surface will improve the ambiance and the lighting.
6) Add detail. Have a bit of time? Whether you have a traditional or modern bath, add a wainscot. The style depends upon the bath. It can cover old tile you can't replace or repair, adds architectural interest, or simply becomes a place to add color. Many home improvement store sell kits that are pre-cut and simply glue to the wall. They will often cut down boards if you bring your room dimensions with you. If you have a long vanity, you might be able to add a tall shelf unit in the center.
7) Change out the doors. Do you have a storage closet in the bath? Add glass to the door and give it a new appearance. Place a puck light inside to shine through. How about adding upper cabinets and use glass inserts? This is, of course, a function of space and scale.
8) Tile it. A more dramatic change is to replace the existing tile with new. You will need a budget, and a tile installer (you can do it yourself!). If the floor tile is in great shape and your walls are painted, add tile up the walls like wainscot. If changing the tile is too extensive a project, try adding a back-splash or small accents like tile around a mirror, niches, etc....