Instructions
Talk to Damien Salt. He'll tell you about his crush, Henrietta, and then task you with collecting either flowers or ice cream to help him win her affections.
Talk to Henrietta, who's sitting by the fountain nearby. You'll learn that she loves flowers but not ice cream. Collecting either item progresses the game, but because Henrietta won't be happy if you give her ice cream, Damien won't leave with her, so you'll have to get flowers for Damien to leave the chest unguarded.
Go to either Mean Street or OsTown; each location contains three hidden flowers. In OsTown, you can find one flower underneath the lawn beside Clarabelle's house, another inside the orange wall near Ortensia's house and a third in the wall near the pool by the Gag Factory. In Mean Street, all three flowers are in the branches of trees in the town square: one to the left of the Emporium and one on each side of the Museum.
Talk to Clarabelle in OsTown to have her combine the flowers into a bouquet.
Return to Damien and give him the bouquet. He'll depart, leaving the chest unguarded. Open the chest and claim its contents.