M.T.
I would get/borrow as many new toys as you can, as they will be more interested in things they haven't seen before. Toy ideas are: lacing cards, stickers, kaleidoscopes, little cars, crayola color wonder stuff, aquadoodle stuff, books with flaps or sound buttons, find it / i spy books, the little fischer price camera that you can see animals in, the character cameras and cell phones that they sell in the toy section at walgreens, little cars and trains, etc.
A lollipop or tootsie pop as a special treat always works well for us! ;-)
We usually try to leave early in the morning when the kids are still tired. And, we usually try to save a DVD for the last third of the trip when the kids are feeling more antsy.
Good luck!