First determine how many miles you are driving. Then decide how many miles you go before you give the a surprise bag. Every 50 miles or every 100 miles, it is up to you.
Lunch bag size sacks
Let each child prepare lunch size paper sack with their own creative designs and their name on each bag. You go to shop and purchase about 5 items for each bag. When you reach the mileage then each child get his surprise bag.
Suggested items: Drink, snack, toy to play with in the car, a small book etc.
Optional. You could let children select the items then you fill the bags and the surprise will be which items are in their bags.