"The beach?" you ask.
"Why not?" asks Celestia. "You and your friends will be here for three whole weeks, and the city of Canterlot is only so big, so I thought that this would be a great way to kick of this vacation."
"Makes sense," you say.
"I already have swimsuits picked out for you and the girls, so you don't need to worry about not being prepared," she says.
"You thought of everything huh, Auntie?" you say.
"You know me," she answers.
With that, you get comfortable and fall asleep with your Auntie Celeste sleeping right next to you. The next morning, at breakfast, Celestia tells everypony about her plans to go to the beach, and they all cheer in excitement. Everypony rushes to get ready to go, and by eleven AM, you were all on the train. You wanted to sit in a booth with Luna, but then...
"Hey, [your name]!" calls Rainbow Dash. "Where're you goin'? Come sit with us!"
You couldn't just blow off your friends just to sit with Luna. You liked her, but you weren't obsessed with her. After almost an hour of idle chatter with your friends, you all finally make it to the beach. As you all get off the train, you see two changing rooms, one for males, one for females. Spike didn't need to change, so you were in the changing room by yourself, which felt rather weird. You look up and see a grate in the wall separating the two changing room. All you needed to do is step up on the nearby stool and you had a clear view of the girls. While you did have a crush on Luna, you did consider all your other pony friends to be a little attractive in their own way. Plus, you thought Chrysalis and Cadance to be pretty attractive, too.
All of a sudden, an angel and a devil appear on your shoulders.
"Do it, bro," the devil says. "This opportunity will never pass you by again! You get to see Luna change AND a bunch of other cute mares!"
"You can't do that!" says the angel.
"Because it's not the right thing to do?" you ask.
"No, because it's too risky!" the angel says. "There are ten mares in there. Odds are, one of them is bound to notice you. Not to mention that one of those mares is Auntie Celeste!"
"Damn, he's got a point," says the devil. "Seeing her changing will fuck up the entire experience."
With your mind made up, the angel and devil disappear from your shoulders, and you go outside to wait for the girls. You hear them coming out of the changing room, and your face turns red when you see them all in their bathing suits. Even Fluttershy, Twilight, and Rainbow Dash with their one piece suits looked pretty hot. Celestia comes out behind everypony else, and she was wearing a pink bikini that was rather revealing.
"Dammit, Auntie," you think. "Why? Just... why?!"
"Wait up everypony!" you hear from inside the changing room. Out comes Luna, who was still adjusting her bathing suit, a two piece that was just as attractive as Celestia's (did you just admit that?), but left more to the imagination. All the other ponies went off on their own, some basking in the sun, other going in the water. Luna, on the other hand, walks towards you.
"Hello, [your name]," she says.
"Hey, Luna," you say.
"So..." she says. "How do I look?"
"You look great," you say.
The princess giggles. "Thank you," she says. "So, how about we..."