I can't leave out messy foods (like yogurt) either, for the same reasons. (Our little scientists!) The trick is to leave out just a few bites of the messier things (berries).
Here's some ideas...
In the fridge:
chopped carrot (cooked) or grated carrot
cole slaw style lettuce (no dressing)
plain cooked pasta
In the pantry:
a variety of dry cereal (cheerios, PB bumpers, corn flakes, etc)
small pieces of granola bar
raisins/dried fruit (only if age-appropriate)