You may get a lot of the same answers on this but here are some good ones that have worked;
1) Fruit-Apples or Bananas
2) Nuts- pick your favorite
3) Beef Jerky
4) Granola Bars
5) Peanut Butter Sandwich
6) Hydration-Drink plenty of water. There are also supplemental powders that you can add to a water bottle that give you electrolytes.
A couple of key points to take into consideration. First, you don’t want anything that is going to be messy. Secondly, the amount you will need will depend on if you are walking or riding. Obviously if you are walking you are going to burn off more than riding.