Oda's artwork: See all the color spreads and reader-requested illustrations on the covers.
Chapter titles: While the titles for the anime episodes tend to be just a summary of events that happen in the episode, manga chapters sometimes can have titles with a lot of thought behind. They can be something simple and recurrent like "Adventure in ..." or "... Vs. ...", but a lot of times they bring an extra insight to the content of the chapter. Chapter titles can range from references to music, callbacks to previous chapters, foreshadowing of future events, reference different things depending on interpretation, etc.
Cover stories: This is the most important one. If you don't know, cover stories are stories that are being told in the cover of each chapter simultaneously to the main story. Most of these stories are not adapted to the anime, so by including the cover of each chapter in the episodes you can watch the anime while keeping track of these events in the same way that you would if you were reading the manga.
At the end of each volume, the volume cover and name are also added, so that you can see where a volume ended and new one will start. The volume format might be the most important aspect to understand how the story is paced and both anime viewers and weekly manga readers via Jump can't really keep track of it. Additionally, manga volumes serve as convenient cutaway points when planning to watch multiple episodes at once.