Ugh, yeah, long flight with a little one. You will likely get up once or twice for diaper changes and that little trip back and forth to the bathroom may seem like heaven for a wiggly 18MO. Just be sure to have a tight grasp in case of turbulence.
Ziploc bags are your friends. Use them to sort out different things to do so you can quickly and easily find things. Need some books? Pick out the Ziploc book bag! Need paper and crayons? Pick out that Ziploc bag! And keep some extra for soiled clothes, dirty pacis, etc. And don't underestimate the fascination of putting things in the bag and dumping them out again.
Puppets! We got the book 5 Little monkeys with the accompanying hand puppet and our kids at that age LOVED it on the plane. Also consider a regular little hand puppet or one that is like a stuffed animal (folkmanis makes some great ones). We used a piglet one to talk and "eat" their hands or hair and the kids just were fascinated.
Sticky notes. Blank paper. All good. Also consider a small travel magnadoodle.
Snacks, snacks, snacks. Sippy cups you fill after security.
Blankies and blanket or two.
Try kidzgear headphones at home (they sell them on
Amazon). They are very comfortable for kids and ours would actually wear them for a while on the plane. Great also for music so they feel sleepy.

Ours still use those today. Heck, I'd use them if I didn't mind stretching them out. They are comfy.
Movies. Consider shorter ones like segments of Elmo or Caillou. Definitely stick with the more soothing variety than something hyper.