Wrap up the various activites into presents (Disney wrapping paper would be perfect). Then write on them instructions. Things like:
Open after lunch
Open when you see the ocean for the first time
Open when you go up a big mountain
Open when you get on I(whatever) highway
Open at midnight
Open at 12:34pm
Open after you have not fought with your sister for one hour straight
Open after you have let you brother choose any CD of yours to listen to he wants.
Open after you pass the sixth Waffle House restaurant.
Open when you cross the Florida state border.
Open after you tell your mother how much you love her and appreicate all the time and trouble she went to to plan this vacation for you.
You get the idea. Vary it to suit your family and trip. This way the goodies are spread out during the whole trip instead of having them rip through them in the first hour. Vary things so that there are treats (Disney fruit snacks and Mickey Juice), coloring books or pages with crayons/pencils, Pez, disposable camera of their own, make your own edible necklace kits, diaries/travel journals, new Gameboy game, homework
etc. And make sure each "gift" is different and exciting as well as something they will spend the next 30-60 minutes doing.
The anticipation of the gifts, the keeping track to see if you get to open another one yet, and keeping it all spread out over a period of time can be quite fun. And if you include a few "be nice to your siblings" incentives its amazing how good they can be!