MiniTheatre

1. Adventure in the Ocean's Navel

The Straw Hats encounter a city in the middle of a whirlpool, called the Ocean's Navel, that is being destroyed by giant monsters. These monsters supposedly protect a treasure capable of granting wishes, but in the process of defeating these guardians, the crew ends up releasing another evil.

2. Open Upon the Great Sea! A Father's Huge, HUGE Dream!

Bonnie and Max, crew members on Pirate Zap's ship, are tired and want to escape, but unfortunately have no money. Three children held captive on the ship happen to overhear them. The eldest, Amanda, who's father was a pro treasure hunter, knows the whereabouts of a great treasure, and with that proposes a deal. They agree, and the five barely escape and make it onto a small island where they meet Luffy and his crew. Unfortunately they were pursued and now Luffy and the others must battle the Bayan pirates and find the treasure that Amanda's father had left for his children. Amanda, who has always resented her father and his love for adventure and treasure, finally understands his feelings.

3. Protect! The Last Great Stage

For many years, Ex-Marine Lieutenant Randolph and his troupe have put on a play aboard his ship. Having lost his family to pirates, Randolph wishes to put on plays that will give courage to those who have also lost their loved ones to pirates. Unfortunately, Randolph must retire soon, and he is now giving his last performance. Luffy and his crew come to see the play, and somehow manage to take part in it as well. But from Randolph's past comes a vengeful subordinate with a diabolical plan who has finally become commander in order to capture Randolph. Luffy and his crew now must prove to the people that not all pirates are scum, and protect Randolph's final farewell performance from the bitter commander.

4. The Detective Memoirs of Chief Straw Hat Luffy

In an alternate reality world that resembles Edo Period Japan, Luffy acts as a member of the police. This is made up of two adventures: in the first, Buggy makes trouble in the town; in the second, a mysterious girl named Vivi appears. This special contains several cameos from characters throughout the entire series. (aired after Episode 253) This special contains several cameos from characters throughout the entire series. A second pair of episodes were aired after episode 290, continuing this story, as part of the New Year's special.

(From ANN, wiki and myanimelist)