Unlike other One Piece video games, One Piece: Grand Adventure one was developed specifically for the US. The gameplay is exactly like One Piece: Grand Battle!. Based on the popular anime and manga series One Piece, Grand Adventure contains more characters, levels, and features, some of which were cut out of the U.S. One Piece: Grand Battle!. The game was also released in Korea, Australia, Spain, and the U.K.
The main mode of Grand Adventure is the Adventure Mode, in which the player must win all 10 levels to complete. The player uses the Going Merry to sail the Grand Line and visit various islands. On these islands, the player will engage in combat against other characters. The players defeated may join your character's crew, or play in various mini-games. In the fights, characters gain experience points and can level up, where the player can level up certain stats of the character (HP, attack, defense, etc.).
Custom Battle: Fight a battle with special rules, such as the ones in Adventure Mode.
Rumble Battle: A fight against support characters. The marines, normal pirates and Kung Fu Dugongs attack in groups. (Support characters such as: Yassop, Lucky Roux and etc. will appear and attack [the number of support characters that appear depends on difficulty setting])
Box Crusher: Destroy 300 boxes before time is up.
Snake Battle: Defeat Nola. Eneru's Priests will try to defend Nola.
Boat Battle: Stop pirates from attacking your team's boat so they can beat the Usopp Pirates.
Snow Shovel: Clean up the roof of drum castle.
Battle Bowl: Get points by throwing Gaimon or your opponent into the hoop
Spotbilled Duck Race: Race against your opponent by riding Karoo.
Source: Wikipedia, "One Piece Grand Adventure", available under the CC-BY-SA License.