Pick your store. This first step is very important, as it will determine the nature and degree of your merriment. A store like Barney's or Neiman Marcus is not going to be very tolerant of any loud or attention-grabbing antics. For that you want somewhere like Kmart or Wal-Mart. But if outrageous pranks aren't your style, go ahead, go for a fancy place. They have escalators! Which brings us to...
Ride the Escalators. In the fancier stores, you have to be a little more discreet, but this is an easy thing to do discreetly. Start by going as far up as you can, and when you get to the top floor, walk around a little before going down again. If you're over the age of six, escalators can get old pretty fast unless you make it a little more interesting. There is a scene in the movie Elf in which Will Ferrel's character, who has never seen an escalator before, attempts to ride one several times before, more or less, succeeding by practically doing the splits on it, attracting the attention of a crowd of shoppers in the process. Re-enact this scene for a laugh, or just go up the down escalator.
Try Stuff On. This probably isn't something you would do in the less expensive stores (unless you were actually going to buy something) because it's just not as fun. But in an upscale department store there's nothing more fun than trying stuff on, and you probably won't be tempted to buy anything since the prices can be pretty steep. Everything is fun to try on but the best things are jewelry, shoes, and wedding gowns. Jewelry goes without saying, and shoes are a natural mood lifter; there's nothing more confidence-boosting than slipping into a fierce pair of heels and strutting around the shoe department for a few minutes. And wedding gowns... well, every girl knows. Although, if you're not actually engaged and you want to try on wedding gowns, it might be a good idea to wear a fake engagement ring in case the fitting room attendant is on a power trip and won't let you try on any wedding gowns unless you "prove" that you're "really getting married." Gosh.
Get Your Make-up Done. A lot of department stores with make-up counters will give you a free make-up makeover. The attendant will even give you tips on what kinds of products to use. They do usually expect you to buy something, but just say something like, "I think I'll browse a little," and then when they move on to another customer, just quietly slip away.
Ask for a Non-existent Product. The key to making this work is to ask for something that sounds like it could be real product. It makes it even better if you make up something really vague, that sounds like it could be one of several different kinds of things. If you're short on ideas, try this: take any word from the first list below and pair it with any word from the second list and there you have it!
Of course, what makes this even funnier is you make up something that turns out to be a real product. If this happens, and the store employee shows you where it is, say that's not what you're looking for and make up a description of something completely different. The hardest part will be keeping a straight face.
Make a Documentary. This of course requires a video camera and at least two people. You can really make a documentary about anything you want. It doesn't have to have anything to do with the store you're in, although that can make for some interesting material. Or you can just film your shopping trip or follow a complete stranger around the store with the camera, secretly, of course. Then put it on Youtube.
Dodge Security. The store security, if they're doing their job, will eventually catch on that you're not there to buy anything, which is, after all, the intended purpose of stores. So you may need to do a few evasive maneuvers to throw them off your scent. Try taking something into the fitting room, not a wedding gown, but something that an average person on an average day might actually buy. Or talk to a store employee. What you talk about doesn't really matter so long as it looks like you're asking them a legitimate question. The security guy will be satisfied that you're a real customer and leave you alone. Then you can return to your merriment.