I'd also like to know this, cannot for the life of me figure this out, anywhere I talk to her doesn't give the option of asking her to the beach. Already got her above 30 to unlock the shower. Is there some hidden prerequisite or something that I'm missing? I would have thought that it would be either the morning or evening in her room, but neither place gives that option, so I'm completely at a loss at this point.
Also it seems that no matter how high I get my VB with her, I can't do the higher sex options with her. Ass eating and Anal both just keep saying "I don't have high enough VB yet" despite having more than 40.