One person playing a guitar is impressive enough especially if that person is able to sing as well. Now take a hit song like "Somebody That I Use To Know" and including all the different musical elements of the song, grab one guitar and five...
Look right into the security camera, and use it as a mirror while you pick your nose. Take up an entire aisle in toys by setting up a full-scale battlefield with G. I. Joes versus the X-Men. Switch the men's and women's signs on the doors of the restroom. Dart around suspiciously while humming the theme from "Mission Impossible." Set up a "Valet Parking" sign in front of the store. Hide in the clothing racks and when people browse through, say things like, "Pick me! Pick me!!" If the store has a food court, buy a soft drink; explain that you don't get out much, and ask if they can put a little umbrella in it. Get cans of cat food and randomly put them in people's carts when they don't realise it. Make a trail of orange juice on the floor leading to the rest rooms. Walk up to an employee and tell him in an official tone, "I think we've got a Code 3 in house wares," and see what happens. Challenge other customers to duels with tubes of gift wrap. Put M&Ms on layaway. Move "Caution: Wet Floor" signs to carpeted areas. Set up a tent in the camping department; tell others you'll only invite them in if they bring pillows from Bed and Bath.