My first thought was something we used to do on our trips from Houston to Fort Worth when my son was 2. Ahead of time I'd take a few of his toys & stow them away when he wasn't looking. He didn't see them for a couple of weeks, so they were more interesting to him by the time we hit the road.
I kept them out of sight in a bag in the front seat & simply pulled a new one out every now & then when he began to get fussy. I also tossed in a few 'safe' (this can be a relative term, I know) household items such as a flashlight or a set of old keys & keyring (which were run through the sanitize cycle in the dishwasher first). Things that normally he didn't get to play with--'grown up stuff'. You can enlist your older children to help monitor those things if needed. We also never went on a road trip without his Magna Doodle. I can't tell you what a lifesaver that was!
New books were always a big hit. You might consider a trip to the library before you leave. Another thing I used to do & still do, actually, even though Jack is now 12 is record my voice reading his books on tape. He LOVED to put on headphones or listen on the car player to the stories while he held the picture book & followed along. At that age, it's helpful to ring a bell or clap on the recording when it's time to turn the page. I'd ring a bell & say, "time to go to the next page". I discovered that this is also an excellent, inexpensive preparation for early reading skills (Can you tell we home school? :)) AND a good way to help toddlers wind down at bedtime after you have said goodnight.
Obviously I don't ring any bells anymore, Ha! Jack is now a voracious reader of chapter books, but I still do read some of his old favorites on tape (like the Little House series) & he often goes to sleep with them playing.