Hi , what kind of style are you thinking of ?
If you like to paint, maybe paint a branch on the wall, add your leaves and maybe some flowers and birds.
I think you mean vaulted ceiling. But regardless, if you like something a bit offbeat, paint it a slightly darker shade of your walls and put up strings of fairy lights. And if that idea doesn't please you, perhaps you could find some stencils you like that would go up the wall above your bed to accent both your headboard and your ceiling. This would carry the eye up to notice the vault. Another idea would be to use fabric that coordinates with your décor and attach it from the "point" of the vaulted wall, carry it down the angled sides to where it reaches the bed then go down in a straight line on either side behind it to the floor. (another way to carry the eye up) Use Velcro strips to attach it so that it can be removed for washing. This would take considerable fabric of course but if you find something you love it could be worth the money and the effort.
*** And if the fabric is washable, such as cotton or a cotton blend, WASH it before putting it up because if you don't, when you take it down the first time for washing it will shrink - not good after spending money, time and effort.