I drove from northern Colorado to southern California a few months ago, and the trip ended up being about 23 hours long. I had both kittens with me and a passenger in my compact car that was stuffed as much as possible.
I had to let them roam around in the car, but they weren't generally obnoxious. I would have loved to have a section of the car caged off for them, but I was moving, so I just didn't have the space. I had a cage for each kitten, and their litter box on the top of the stuff in the back seat.
I had them in the carriers until both people were settled, and then I let them out. They moved around for a bit, and if I'd been alone, it would probably have been bad. My passenger was able to comfort them and keep them out of the windshield, though. I had their harnesses on them so they were easy to grab. That was important when Bella decided to nap under the clutch in the middle of Utah. Rowan got his harness off, and chucked it into the front seat just to show that he didn't need it, and he was angry that I put it on him, but we were only about an hour away from our new house, so I didn't bother to pull over and put it back on. I just stuck them both back into carriers for the walk inside.
Neither of them likes their own carrier much, but Bella doesn't mind being in Rowan's, and he doesn't mind being in hers. Don't ask how it works, but it just does. Rowan and Bella napped for a while in his carrier, but he spent most of the trip in the litter box. He hasn't ever expressed a desire to be outside ever again, so we let him stay in there during stops when we got out. We stuck Bella in a carrier, though. I think that having their favorite blanket out for them to sleep on really helped, as did their towels on the bottom of the carriers. Bella was much less upset when we could cover her (she's a real cave kitty).
Bella cried a lot about the altitude changes. At least, I assume it was the altitude changes. She mainly cried when my ears were hurting too, because I had a nasty head cold.
I didn't really 'train' them with little trips. Bella just flat out hates the car. Rowan is just fine in the car, as long as he's not locked in a carrier. I don't really think that I could ever convince Bella that being in a car is fun. She doesn't like the engine noise.
I had water and food avalible for them the whole trip. Neither of them pukes unless they're fed really poor-quality food, so I didn't even think about car sickness. Rowan ate and drank (and used the litterbox), but Bella didn't eat, and I don't know if she drank anything.