On the Main Street of Numara on the dock in front of the Artists' Salon there is a girl standing in a Gondola. She'll tell you that she is waiting for her date. Complete the 2nd part of the Lonely Chuchu's Treasure quest so you can enter the back door of Nalia's Item Shop. Take the northern exit. Talk to Cocktis who is standing on the gondola twice. He will bring you a chest so you can get the item inside. Go back to the girl who now is with her boyfriend. Talk to them and they will go get another chest for you.