While still at home, place raw eggs into a small saucepan. Add enough cold water to the pot to just cover the eggs. Bring to a boil (uncovered), then immediately turn heat off. Cover eggs and let sit in water for 20 minutes. Rinse in cool water and peel.
Smash peeled eggs with a fork in a medium bowl. Add mayonnaise, mustard, celery, salt, and pepper. Blend well. Let cool to room temperature and place in a zip top bag. Refrigerate until you assemble the sandwiches.
At the campsite, toast 4 slices of bread by preferred toasting method (see above). Scoop half the egg mixture onto one piece of toast and top with another piece of toast. Repeat for second sandwich. Enjoy your easy camping sandwich!