As others have suggested, you’ll probably want to pack a combination of movies, books and audio.

I don’t know what kind of media you normally consume, but here’s what has worked for me and others in the past:

• 18 hours is a looong time. Your attention span and interests may vary wildly from one minute to the other.
• Download some series in addition to regular movies. 35-50min chunks are much more manageable with a reduced attention span.
• Get some e-books about various topics, ideally some fiction and some non-fiction. Again, some short stories can’t hurt. Or some entertaining science book such as What If? by Randall Munroe.
• Audiobooks are a great way to be entertained with your eyes closed. You can actively consume them or fall asleep while listening.
• Include some movies/books that you already know but don’t mind watching/reading a second time.

Only maginally related to your question:

• By all means, pack a bottle of eyedrops. My personal recommendation is Bepanthen Pro if you can get them where you live. A bit on the expensive side but absolutely worth it.
• If you plan to bring your own comfy headphones, consider getting one of those Bluetooth transmitters with a headphone jack so you can watch movies on the built-in screen with your own headphones. (Pick one that can handle most planes’ headphone plugs that use two jacks.)