My husband can't fish because a bobbing boat in water can get his stomach churning. I took a cruise about five years ago (never again!) that was excruciatingly miserable because of motion sickness.
And as for car rides, your story was me when I was little...from about 2 to age 5. I remember my parents and grandparents taking me on a car trip to visit our extended family in California. Just like the famous song, we d
drove along "Route 66" just for "kicks".
About that time, my grandfather bought a brand new Cadillac just for the road trip, and sadly I succeeded in ruining the luxiurious interior in no time. There is nothing worse than driving across a dessert in a smelly, confining car! Cadillacs are big inside, but not big enough when you get sick in them. Even with air conditioning it was unbearable!
I think I got sick about 4 times during the 3 day car trip from Minnesota to California. The worst round being when we crossed the Golden Gate Bridge in San Fran.
What I remember seemed to be the trigger was movement and the sensations I felt as the car would glide over different surfaces on the road. When going over the bridge, I remember having a hard time seeing out the window because I was so little (before car seats for toddlers!) so all I could see were the bridge beams and lights passing the window at high speeds...making weird shadows in the interior of the car. Since I was just a kid, I didn't have the sense enough to look away when I started to get woozy. Before I knew it, I my head was spinning AND my stomach was turning.
I agree with distractions like a dvd player, or activities that will keep her looking in different directions instead of focusing in one direction. Try car bingo for instance. She'll look for specific objects and not focus on things just passing by.
Also take lots and lots of pit stops. On my famous trip, we made very few stops. I think it was too much motion for too long of a period. It may be annoying but every 45 minutes to an hour, take a five to ten minute stop so she can stretch and get her land legs back.