18 months is a hard age to travel with. Best Wishes! They don't do well out of their element. They are a danger to themselves and are constantly on the move. They don't like to be carried, don't like strollers, don't want to hold your hand.
I don't advocate drugging your kid but you might want to call your pediatrician for some recommendations on safe sedatives like Benedryl or Dramamine. Also for some tips on dealing with constipation.
For the car ride. Expect to stop every 1-2 hours for a few minutes to stretch her legs. Have a ball or something handy for her to play with. Stop at Mc D's, Chick Fillet or the like for the playground or a rest stop for the lawn.
I don't like in-car DVD players. They can cause sickness, crankiness, insomnia, and lack of imagination. We use audio books and music and have never had any complaints from our kids. The audio books we checked out from the library. We found that older kid books like Lemony Snicket worked fine for the babies too. The funny words and voices were mesmerizing. They also helped them fall asleep.
Also, our favorite car toys; mini-doodle pads, noisy button games, small board books, magnetic books and pretend cellphones.
Snack and drinks are a must. We made a few small pb&J's to go and learned to put our snacks like goldfish crackers in cups rather than baggies, snack boxes, or bowls. The fit in the cup holders. Watch out for juice boxes random squirts. I took a pocket knife and transfered juice box juice into sippy cups. I couldn't find one of those nifty juice box holders anywhere.
I kept a 1 L bottle of milk on ice in a lunch box on the road and in the room for those late night requests.
You might consider utilizing a harness/leash thing to prevent her from darting off in a crowd. Also, write your phone number on her arm with a ballpoint pen. There is a DVD by Sesame Street where Big Bird gets lost that all kids should see.
If you can, take an MP3 player with speakers with her favorite lullabies to listen to at bedtime to make her feel more at home in the cabin and block out some of the noise.
You might consider packing her a life jacket. They might be hard to come by once you get there. Don't forget her sunglasses and hat.
Have a great trip!