Well Thanksgiving is over, so we are in the "official Christmas season". Rather than bitch, let's have some fun with this season. People are at there worst during this season, so it seems only right to screw with them. How can we use the elements of this season to make this season fun?
In the overcrowded parking lots there are many way to have fun. Organize a card game that take place in a parking lot. Bring a large folding table (or several) and take up as many spaces as you can with your friends gambling. Make sure someone in the group buy some gum form a store connected to the parking lot, so you can't be accused of loitering. If gambling isn't your thing, you can do the same thing with a game like "Magic the Gathering" or "Warhammer". Do whatever you like at the tables, the idea is to take up as may spaces as possible to make the parking situation that much worse. Keep in mind nothing about this is illegal, it is just a massively asshole like behavior.
Now if you actually must go into a store you need not limit yourself to only shopping. You could always play with the placing of prices on the shelves, or the location of merchandise. Either will have the same effect. While people backs are turned you can take the time to remove or add items from/to their cart. Of course if all of that is too much work for you, there exist other options. Why not eat some Mexican food and then go to the most crowded part of a store and let the pressure release. Then there is taking up the time of sales people looking for products you know they don't have, and asking questions they can't answer. Not only are you wasting the time of the sales person, you are wasting the time of every actual paying customer.
There are many thing you can do. Be creative, and know the law. You will be amazed at how far you can go without breaking any laws.