1) The easiest activity that we have done, with some measure of success is a cookie sheet and magnets. I group magnets by theme in little baggies and give one or maybe two bags at a time to my backseat driver. I also got my hands on some plain white magnet sheets and using favorite stickers I made some personalized magnets. I'm looking forward to getting the poetry magnets once he can read and letting him create some silly stories too. I have also started a magnet collection from places we visit which I bring out during subsequent long car trips. It's fun to remember where they came from.
2) Another simple thought is a post-it note pad. Give them a pencil and let them draw or show them how to stick it on the window to make artistic designs.
3) I see that glow sticks/necklaces already made it on the list, I was going to add mini flashlights to that group.
4) Lately we have gotten into audio books a lot more (probably because our TV is still "broken" and this seems to be the new replacement). I've discovered free children's story podcasts on iTunes and can recommend the following:
Storynory, My Baby Monsters, Pinky Dinky Doo, The Story Home, BigStoryTime. You can also access some free, and not free, stories online at these sites (I haven't checked these out too well though): storynory.com, audiblekids.com, candlelightstories.com, linkslearning.org, audiostoriesforkids.com, bigstorytime.com. Of course you can download whole books from iTunes as well, or simply check them out from your local library. Along this same vein, look for some new kids songs free on podcasts or from the library.
5) Here are some wonderful online resources for some fun travel ideas I found on FamilyFun.go.com:
10 Ways to Make Family Travel More Fun It's four pages, so don't stop after page one. I love the Travel Fairy and Sticker Detective ideas! Plus #8 is a real winner in my book.
Travel Games: Inventing, Finding and Counting You can click the links provided to get further ideas for other types of travel games too.
Travel related Search A list of more ideas than I had time to read.
Have a great trip!