When your containers are newly painted and fully dry (wait at least 24 hours after spray painting), then you can start embellishing them with washi tape. I started with my rectangle shaped cookie tin and added washi tape just like it was ribbon on a wrapped gift.
When the tape was lined up, I used the X-acto knife to make cuts where the cookie tin opens up. I also trimmed the washi tape on the bottm right at the edge of the container for a nice clean line on the bottom.
You can use similar materials and make this DIY Lunch Box Treat & Note Holder From An Altoid Tin