Our first trip away wasn't as long as yours, we went 3 hours away for one night when DD was 6 weeks old. We took very similar stuff to you, except we took a portacot for her to sleep in at night, and her pram with basinette to sleep in at the party we went to.

We didn't bother with a play gym/mat, if DH or I weren't holding DD there were plenty of rellies looking for cuddles!

In two weeks we're going two hours away for the night, and this time we won't bother with the portacot as she slept with us last time due to the freezing cold. This time she'll sleep with us or in the pram basinette.

As for the drive, on the way there we left when she fell asleep the first time that day - about 8:30am, and the car ride kept her asleep the whole way, she was great. On the way home, we left after lunch and she'd already slept a fair bit that day. So she didn't sleep much and just didn't want to be in the car. We stopped a couple of times and she was fine.

You might also want to consider where the sun will be coming in through the windows, we found it hit DD in the face from one of the back windows opposite her, not the one right next to her.