This is an age when kids seem extra restless whether they're in the car seat, the stroller, the high chair or anywhere they're strapped in. Make sure he's as comfortably (but still safely) situated in the seat as possible, and be sure you have the right seat for him; if he's a particularly big boy, for instance, you may want to look at other seats that would fit him better. And the toys, snacks and music are all good ideas but just be aware that he won't ever be content every minute -- try to ignore him when he fusses so he doesn't get the idea that you react whenever he fusses. It's not necesssary that he feel entertained every moment.
I am not a fan of the DVDs in the car idea. I know lots of parents consider them a godsend especially on long trips, but I think they teach kids that they always need a screen in front of them to pass the time. And it doesn't just end up being for long trips -- friends who have them say that though they try to reserve the car's DVD player for longer drives, the kids quickly learn to fuss for a DVD on even the shortest errands, so they can't spend even 10 minutes in the car without wanting something moving on a screen.
He's plenty old enough for lots of music CDs (both kids' songs and your own music; you'd be surprised how early kids like pop or rock, and be sure to change CDs often so he doesn't get bored with the same old ones). Play simple story CDs or cassettes you can get for free from the library, etc. Some libraries have younger kids' board books etc. with accompanying CDs, so he can hold and look at the book while the CD plays. I would try giving him soft puzzles he can hold on his lap (if he tends to throw things like puzzle pieces, which could be hard and hit and distract you, don't do this, or find soft puzzles). Or reserve a special, new toy that ONLY "lives in the car" like those dolls that have buttons/zips/snaps etc. for him to do and undo, and so on. New stuff that keeps his hands busy. And of course talk to him and play "I spy" and show him things out the window all the time -- it actually will help with his visual acuity and color identification, word association, etc. as he gets older. My daughter used to love to spot shapes, colors and eventually words outside the car. He's too young for most of that but the more you say it to him, the sooner he'll pick it up. I would add that my daughter really used to look forward to long (four-hour-plus) car drives to see family because she knew it meant long, uninterrupted stretches of time to listen to story CDs. Good luck and have fun in the car!