X Marks the Spot - S1.E17

While playing a game on her smartphone at the beach, Sunset Shimmer discovers a message in a bottle. She shows it to Twilight and Pinkie Pie, and they realize it's a treasure map that looks like the beach they are on. When they follow the map to a treasure chest under the pier, they discover a second message in a bottle that's encoded and a decoder ring. Sunset decodes the message using the decoder ring, and it tells them to follow a trail of seashells to an "X" at "the end of the rainbow". The "rainbow" in question is revealed to be an ice cream truck, and the "X" turns out to be a Rollin' Sushi Truck. The sushi truck attendant congratulates the girls on solving their promotional treasure hunt, for which they win sushi at a discounted price. The girls are a little disappointed at not finding real treasure, but they enjoy the sushi anyway.