From Bulbapedia, the community-driven Pokémon encyclopedia.
Fixing Pichu's Ukelele
After you get back to Booker's house, he notes that Pichu has broken its ukelele and sends you and Nick out to get the materials to fix it. The materials are on Dolce Island in a hollowed out tree stump (near the bottom of the area after you get washed up). Nick goes in and tries to get them, but accidentally disturbs six Sunkern who are in the bottom of the tree. Use Poké Assists to make this capture a lot easier. Afterwards, you bring the materials back to Booker, who fixes the ukelele.
Befriending Celebi
Head over to the stone of time, where there is a glowing orb. Suddenly, it starts moving and scaring all of the villagers away. You go to capture it, which reveals itself as a Celebi. Avoid its attacks and keep looping it to get a victory. Afterwards, it takes you back in time where you meet Ravio. Ravio is unhappy with Celebi, and tells it to take you back to your time, to which Celebi complies. Then all the villagers ask about your disappearance before you continue on.
