If it is possible, frequent stops are an absolute must...about every two hours. If it is possible, travel in the evening when your toddler would normally be asleep. Other suggestions include a portable DVD player with lots of Disney movies, Magna Doodle Boards, fun and interesting snacks to eat, view finders, any new small action figure toys for boys or small Barbie like toys for girls, preschool matching games, a small play food set, legos that are appropriate for 3 year olds...etc. It would also be helpful to have a portable lap board that your child can place on his/her lap without it falling.
I hope that this was helpful. If so, please ACCEPT. BONUSES are also accepted in appreciation for my efforts.