Hi M.,
We let our 3 yr old watch movies (both from our collection and from the library) and a couple of weeks before our trip I go to the Dollar store and a few other stores (I try to go by myself) and I pick up about 12 new things and I put them in a large bag and seal it with binders clips and out of his reach.
During our trip (I wait until he gets fussy and then I start to pull out something new about every hr or 2). When I give him a new toy, the "old" one gets put back in the bag until the trip home.
On our last trip I did slurge and he got new magnetic Thomas train book from Toys R Us, 'I Spy' books from the library, new coloring books, a lap size Thomas set with a clock and a bridge, window clings, viewfinder with Pooh and Cars, a fire chief playmobil (they have small pieces, so I really watch him), small trucks and cars, Cars magna-doodle, magnetic Cars book from Target, a small toy that he can push buttons on (I got him a Simon-but that didn't work on our last trip, he's still too young). Then during our drive I take a large plastic container lid, flip it upside down and let him use that as a table, which is handy because it already has a 'built in lip' to catch things.
We drive during the day, stop as little as possible (he gets really fussy when we try to put him back in his car seat after too many stops) and we stay at a hotel with a pool, half way through the trip. Our trips are usually about 12 hours and this has worked for us.
A great website is.....www.MomsMinivan.com/toddlers.html
Good Luck :-)