H.E.
I'm going to keep checking this post as I'm hoping someone knows of some neat things like you're asking for! I'd love to get something similar for my son, who is two. ;)
My son will now keep some things with him and play on car rides, but also sometimes throws things down and then bugs me constantly to get them for him (which I can't do while driving).
I did put a basket next to his car seat (in the middle part of the car) and filled it with some toys. He can only reach the very top stuff, and sometimes that can get annoying (when he wants something in there out of his reach)! Ha ha!
I did find a little "caddy" that hooks on the side of a car seat ... you can view it at http://www.onestepahead.com/catalog/product.jsp?productId.... It can hold some (small) items for children to occupy themselves with during rides. One Step Ahead has some neat (but sometimes expensive) items for little ones!
For a larger pouch to hold things, check out http://www.onestepahead.com/catalog/product.jsp?productId.... And, they also have a table and footrest combo for car seats at http://www.onestepahead.com/catalog/product.jsp?productId.... And, they also have just a tray (without footrest) that you can check out at http://www.onestepahead.com/catalog/product.jsp?productId....
I'm not sure how I got my hands on their catalog a while back, but they always seem to have some unique and handy items for sale. You can search their Web site (www.onestepahead.com) for anything you might be looking for. Good luck!