Go up to a costomer and follow them around, saying that everything their buying for dinner is WRONG. Then, lead them to the dog food isle, and say "This is the right food for you," and stuff some in their mouths.

(Not sure if this has been taken). Stick a remote on a Barbie doll and start talking. 

LOL!  Take all the ice cream and somehow spoon it out and slop it on the floor.  after that, take some chocolate drizzle, and pour it all over the ice cream.  When someone comes by, say 'Want some?  it's only one dollar!' and if an employee comes by, say 'OMG!!!  Extra ice cream on the house for you!'

oooh I have a great one!
(real story)
-Find a walmart (preferably out of town)
-go in with a few friends (were talking 5-6 people) and take over nearest gazebo display
-take one of the big blue bouncy balls (ya you know were they are!)
-bust out an iphone loaded with music (XD)
-blast it full volume and have a party in the gazebo (throwing big blue ball, singing...er screaming along wit music etc...)
-get kicked out within 5 minutes of being in the store

ORRR (another personal experience)
-have 5 + friends go to walmart (out of town!!)
-take a shopping cart
-have as many people as possible cram into that shopping cart
-a walmart employee will spot you soon and order you to stop
-run away from angry employee trying to do his/her job (with heavy shopping cart full of friends)
-a paparazzi of employees will soon form
-ditch shopping cart and run out as fast as you can

XD good times...

((Lol smokey!))