I love the frozen milk idea.
Once in grade school a mom made a sandwich with ham and a piece of cardboard (wrapped in plastic wrap) for her son's sandwich. She hid the piece of cheese in the bottom of the lunch box.
A few years ago I had DH bring a brown paper bag to work filled with treats. He just left it on the lunch room table. This was a few days before the 1st. Everone at his place just helps themselves to anything.
Then on the 1st, he dropped off the bag on the table. This time we had a note in the bottom saying "Why the He$$ are you looking in this bag? It's not yours?" After the first few people looked, anytime someone walked over to the lunch room table everyone would watch to see their expression.