Hang wallpaper around the area where the sunken tub is or install wood paneling. This will help to differentiate the sunken tub area from the rest of the space. The wallpapering will make you feel like you're in an old-fashioned but elegant hotel, and the wood paneling will make you feel like you're in a luxury wood cabin.
Frame paintings and prints in vintage gilt frames, and hang them so that they're at eye level when you're in the tub. This will give you visual adornment to admire when you're soaking in your tub.
Install a small wall shelf at the head of the tub, placing it at eye level. Place decorative trinkets, like exotic soaps, bath beads and figurines, on the shelf to add more texture and dimension to this area of the bathroom.
Select plant pots that match the frames of your pictures and prints or complement them well. Install plants in these pots that are ideal for the tropical climate of the bathroom, but that also give the space a regal atmosphere, such as African violets, Boston ferns or potted orchids. Create a cluster of plants near the head or foot of the tub.
Place a furry sheepskin or faux animal skin rug near the tub as a rich material to sink your feet into after you take a bath.