Pokémon: Pikachu's Vacation(1998)

Pikachu no Natsuyasumi

Ash and his friends arrive at a Pokémon-only park. They set all their Pokémon loose and, after Ash tells Pikachu to watch over Togepi, they enter for a day of fun and relaxation. Unfortunately, it's not going to turn out the way they had hoped.

(Source: Serebii)