Definitely do not miss the must-do classics:
- The Haunted Mansion
- Pirates of the Caribbean
- Big Thunder Mountain Railroad
- Space Mountain
- it's a small world
- Happily Ever After (fireworks)
The Magic Kingdom is certainly not at the level of Disneyland, (which has the vastly superior versions of Pirates and Space, plus Star Tours, Matterhorn, Indiana Jones Adventure, and Rise of the Resistance among much else), but MK still has enough that's worthwhile. And if time permits, almost everyone enjoys the relaxing Liberty Belle Riverboat ride and the Seven Dwarfs Mine Train (at night, when the line is shorter).
As other posters mentioned, have you considered visiting the other parks? The best of WDW is really outside MK, IMHO. Animal Kingdom has the incredible Flight of Passage, Kilimonjaro Safaris, Expedition Everest, and DINOSAUR attractions. Hollywood Studios has Rise of the Resistance (and the rest of Galaxy's Edge), Tower of Terror, Star Tours, Rock n' Roller Coaster, and Fantasmic! EPCOT has Cosmic Rewind, Soarin', Mission: Space (the Mars side is fun, as long as you don't get easily motion sick), LuminoUS, and of course all of World Showcase.
As others have said, so much of Disney is about relaxing and taking in the environments. Take your time on Main Street, walking along the river in Frontierland, enjoying the nighttime lighting in Fantasyland and Tomorrowland, etc. And the same goes for the other parks: Animal Kingdom is stunning practically everywhere, Hollywood Studios's Hollywood-themed areas and Galaxy's Edge could be soaked in for hours, and of course World Showcase at EPCOT is at another level...
