One of the most important things to do when designing your bathroom is to consider the principles of universal design, which means that even if you have some unique ideas for your bathroom, it ought to be easily accessible and safe for all users.
You can design by the usual method of making a scaled-down drawing and the required appliances, or you can use a computerized design service which allows you to draw up a virtual plan and drag and move accessories around it.
If you already have excellent plumbing fixtures, there is little point in overhauling them and you could instead try a fresh coat of paint, new wall-paper, or mosaic tiles on the walls or floor.
Use natural colors or soft pastel hues, with maybe a few touches of brighter primary colors. Apply a lighter color to the bathroom cabinets. Draw attention to the ceiling with decorative color treatment or add decorative beams.
For floors, go for durable materials like stone or ceramic. These come in a range of natural colors and textures and the ceramic tiles, of course, in a range of patterns as well. You can also try vinyl or wood flooring, but these might require considerably more maintenance in a bathroom.
Instead of the usual bath-tub, you could go for a whirlpool tub. Get a flexible spray-hose, and install a water-temperature control device so there's not even a remote chance of scalding yourself. If you have a shower, consider putting in a seat for a more relaxed bathing experience.
Display collectibles like shells or stones or china figurines on a glass shelf and put up framed paintings. Add a wicker table to hold your collection of rolled plush towels, soaps, bath salts, and pretty toiletries. Keep aromatic candles near the bathtub and on the vanity table.
Get towels and shower-curtains in different colors and funky patterns that complement each other and go well with the overall décor as well. Add a chair. It'll come handy when you want to apply make-up, dry your hair, or just sit still and think. Put in a couple of potted leafy plants or some fresh flowers. Think creatively.
Flowers don't have to be displayed in ordinary vases, you can use interesting-shaped glass jars or cans. If you have a nice window with a window-box, you can plant some bright flowers there. Hang up a small chandelier in the center of the ceiling. Scatter non-slip bath mats around the bathroom and place a bright rug near the vanity table.
For something nice and extravagant, try getting a towel warmer to keep your bath towels warm on cold winter days, a radiant heating system under the floor to keep your feet warm and a built-in stereo system to infuse music into the gloomiest baths.