"You are the Prince," Riku pointed out. "You can give me whatever you want. I look forward to whatever you decide to do for my birthday. It has been a very long time since I've celebrated it." Fifteen years, infact.
"Alright." Riku picked up the Prince's soap and added a little more to the sponge before running it over the Prince's back, washing him slowly and at his own leisure. He directed the Prince's head to his shoulder while he worked, just enjoying the warm water, wet skin, and closeness.
When he finally had the Prince nice and clean and his fingers were beginning to wrinkle, Riku reluctantly let go of the Prince and climbed out of the bath to open a towel for him.