A.S.
I agree, adding fairytale creatures (mermaids, fairies, frog prince, etc) to your pond and choosing one color to paint the remaining walls would be adorable.
In case you aren't artistically inclined they make wall paper cutouts in all sorts of fun shapes. Wallies is a popular brand. You could just purchase and paste your new characters on the mural. http://www.usawallpaper.com/walstic.html
And then matching your bedding up to a color in the mural would pull it together. You can even take pictures of the mural and get them printed onto throw pillows. One thing I did in my daughter's room (also 3) is use a quilt in coordinating colors without any sign of her theme (horses) on it. This way if she decides she could care less about horses in a year or 2 I can just change her accessories.
Good luck!