Today my eldest son has sports carnival at school. I made the same theme for his bento. I prepared ham & cheese sandwich written with "sports day" from eggsheet, and I put whistle (eggsheet, nori, white crab stick, and black sesame seed) and running shoes (eggsheet, red crab stick, and nori) and decorated with spring onions.
For the side, I prepared sausage, broccoli, and corn kernel.
Here is similar tool I used for this post :