When my daughter no longer needed the diaper bag we went to a small child's backpack. It has Dora on it and is sized just right for her to carry which can help when hands are lacking or when loaded up with lots of other stuff.
In it is: Anti-bac hand wipes, small pack of reg. wipes, two complete changes of clothes, a pull-up (just incase we are going for a long ride since she still doesn't wake to go and sometimes has accidents when sleeping), and a snack (crackers, granola bar, cereal bar, cereal, gummies, etc.).
It's light enough for her to carry. If we are going for a really long trip (like a whole day trip), I usually put in 3 changes of clothes just to make sure. You never know and I always like to be over prepared. She rarely has accidents and usually it is due to her falling asleep on a long drive. She usually needs clothes to change into because she made a mess eating or drinking, LOL! :)