Stage fake Pokemon battles with your friends using stuffed animals. (For added effect, get a Pokemon soundboard on your phone, so there's sound effects too.)
Tell customers about "the giant rat you saw behind the deli counter that snarled at you once."
Sing the My Little Pony: Friendship is Magic theme song, or Sunshine Lollipops and Rainbows, or anything like that, as loudly as you can th whole time youre in the store.
When you see someone buying Goldfish crackers, ask them how they can keep all those fish as pets. When they tell you they were planning on eating them, act shocked and horrified.
Make a fake High School Musical music video. Bring your friends, and have a really dumb dance all planned out. Then have someone film it. See if any customers join in singing.