Crackers (Ritz are what we usually have on hand).
Sliced Cheese (pre-sliced, or slices from a brick)
Meat (lunch meat slices, hot dogs)
1. Cut cheese and meat into 1" squares
2. Place crackers, meat and cheese together in a sandwich box*
3. Allow kids to assemble their own "sandwiches"
*I have been known to cut wax paper into squares and fold separator boxes for the different sandwich elements. If you're interested, look here.