My daughter was the same way and I dreaded each time I would have to take her to Childrens Hospital of WI because it meant long painful rides for both of us! We ended up downgrading our car and got lucky that the one we bought had a DVD player in it. WOW- talk about total change in long car rides!!! I do not put it on though if we are only going to be in the car for 30 minutes unless she is overtired and I know it is the only way to get home and keep my sanity! lol
I was like you and did not want her to watch just mindless cartoons. I looked around for different DVDs and found that Baby Genius has some great ones that are fun as well as educational. Even Elmo is educational! Most DVDs are geared towards education as well as fun. Also there is Baby Mozart (my daughter does not like these though) that is very educational. Her favs for the car is her Baby Genius, Elmo and Barney. I also have a few mindless ones like Boobahs and such but she prefers the other ones. I even went and bought Smurfs (ok, I bought it for me but ya know! lol )but she is too young to get the meaning behind each story yet!
Other than a DVD player my daughter LOVES music... preferably Hip Hop/Rap! She loves the bass and will dance to it. I figure what difference does it make if it is Mozart or Snoop Dog (or whoever) as long as she is enjoying it and able to move and find comfort in it. She will actually sign to me to turn it up (she is delayed in speaking) and will keep signing until my eardrums feel like they will burst! lol I have tried to turn her on to country or even rock but she will not have it! lol Oh well!
Sometimes we also sing songs (her making noises) or I tell stories and nursery rhymes or play watch for certain things. I will say watch for the cow and when she sees it she will scream (ear piercing) so I know she found it. It helps if you know what might be coming up. With your son you might be asking right now and pointing to it and clapping saying We found it, but eventually he will catch on especially watching his older brother find things!
Remember that you are trying to have a nice road trip and if a DVD player is what keeps the trip being nice it is well worth it! I know my parents used to sit us in front of the tv every saturday morning for 6 hours while they slept and I feel I turned out fine... for the most part anyhow!
Good luck and many blessings to you and yours!