Go at 3 in the morning and take all the bikes off the racks and form a train around the store and use silly string to spray ppl
Lol!! My cousin through a basketball at one of the cashing machines and broke it! Luckily it was late and there was nobody around!!
Hand out free candy to little kids dressed as a creeper Bonus if you drive a giant van that has free candy painted on the side
Or, walk up to the automatic doors and when they open for you, say, "Holy crap! We don't have these in my home planet of Zorphoron!" Or some planet off of a scifi movie.
Say things that have happened like a man on the moon, women in congress like they have never happened when you are checking out. If they correct you claim that you predicted it and that you are a wizard