There is an entire tradition of gifts wrapped in pretty pink paper in Harvest Moon. The 'Amazing Bathing Suit' in Rune Factory Frontier is an item that your character can make at the Crafts Bench. When completed, it will appear in a gift-wrapped box in your Rucksack.
Neither the lesser Cute Bathing Suit nor the Amazing Bathing Suit will be accepted by an Eligible Girl unless she has reached 8 Hearts. Whenever an Eligible Girl accepts either the Cute Bathing Suit or the Amazing Bathing Suit, she will promise to wear it for you.
Here is a video that reveals the manner in which any Eligible Girl keeps her promise:
There is an old saying, rather impolite, to the effect that 'To Assume makes an Ass out of you and me'. Once I had the necessary skill to create the 'Amazing Bathing Suit', I offered that to the Eligible Girls instead of the Cute Bathing Suit. I published the video showing the results and then, almost as an afterthought, gave Eunice the 'Cute Bathing Suit' I had created for the part of the video that showed the recipes for the two Bathing Costumes.
I went to the Beach, not expecting anything different from previous visits, but to my amazement and delight, when I surprised her in the Changing Room, she was changing into the Cute Bathing Suit! I therefore had to remove my video from YouTube in order to remake it with the Cute Bathing Suit result... (It saddened me to be obliged to delete it as some one had given me 5 stars for it, but I hope that the new video will meet with as kind a reception.)