We did our first trip with DS at a similar age and I took pretty much everything! It was a bit OTT, but I'm very much a JIC kinda girl, so I brought things like the thermometer and panadol in case he came down with something. If you're not going to take his playmat, then I'd take a sheepskin or something so that you can pop him down on the floor and let him have a stretch for a bit.

I wouldn't worry too much about the car trip. Ours was only about 3.5 hours and DS slept the whole time, which I didn't expect. But we'd just planned to pull over or stop somewhere if he needed a feed along the way.