If you go to the party store, dollar store, the party section of Walmart, or (right now) the Easter section of your drug store you can find cheap little stuff like rings that light up, games that involve moving a little ball through a maze, a chick you hold in your hand that cheeps, plastic animals, etc. Give them out one at a time when things get boring.
Get some Colorwonder markers (the fingerpaints are kind of messy) and paper. When your little guy drops the open marker between his car seat and the seat of the car, it won't bleed all over everything (maybe it will bleed, but you won't see it.
Get a Klutz or Chickensox book for your dd. They come with all the stuff you need, but look over them because sometimes they're messy or require more space. There's a book for making animals and bugs out of pipe cleaners and pom-poms, one that comes with rolls of colored tape to complete pictures, and some to make girly stuff like bracelets and hair accessories.
Get a "grabber" for your the little guy because he won't be able to pick up anything he drops. This thing will help him get things off the floor and keep him entertained for a while.
http://www.amazon.com/Robot-Claw-Grabber-by-Toysmith/dp/B000F4G5KK 
