I think it would be really fun to turn off a stereo or something and put it to the top volume so when someone turned it on it would blast in there face =P
Go to the clothing depo and put the hangers so that they are balancing on the rack instead of being hooked on (only works in some stores) so that when people brush against them they will all fall. >
I saw this really funny video of a prank in a german matress store. here's the link.
http://www.youtube.com/watch?v=zixFTJ439PI
another good prank I saw was to have a changeroom curtain that kept rolling back up after it was pulled down. I can't post that video though cause it has adult content. if I were to pull a prank in a store I would most likely get one of those universal remotes and mess around with the TVs in the electronics section. that or i'd dye all of the shaving cream green.
one time me and some of my friends went to the local mall and pretended to be manicans(the plastic models dressed in outfits). and we would just stand there perfectly still. whenever someone walked close by we would jump at them or whisper to them it was hilarious. ( we were in the display cases XD)