AVOID SUGAR SNACKS. It only increases their desire to move around.
It's great to have a series of little "surprises" packed away that you can bring out at intervals.
Invest in some of the Crayola markers that only color on the special paper. Plus your car will not end up with color markings from the normal markers. Small toys are fun. One thing that is really annoying for parents, but if you can get past the noise. They make 10 dollar hand held video games. The kids think they are actually playing, but just moving the players. Battleship, Hangman or solitaire.
Baby wipes seem to seriously entertain my children. Maybe purchase some no-name brand if in a crunch for entertainment and just let them play, play, play with the baby wipes.
Not sure you would want to leave them unattended but if you go get the 1000 little stickers, and when they are fussy give them a whole sheet. They will be surprised to just pull them off. Maybe get them a little book they have to put them on. They might end up on them or the car but what fun. You might put the book in a zip lock bag with a sheet of stickers. It will be handy in a pinch.
Good luck have fun!!!