Celebrate Buy Nothing Day!
A 24-hour moratorium on consumer spending - participate by not participating.
Activities to consider:
Credit card cut up
Volunteers stand in a shopping mall with a pair of scissors and a sign offering a simple service: to put an end to extortionate interest rates and mounting debt with one considerate cut. Be careful though: in some first-world countries, carrying scissors in public can get you arrested as a "terrorist".
Zombie Walk
The cheerful dead wander around malls, marveling at the blank, comatose expressions on the faces of shoppers. The zombies are happy to be among their own kind, but slightly contemptuous of those who have not yet begun to rot.
Whirl-mart
This activity has the advantage of being most likely to piss off security personnel. You and nine of your closest friends silently drive your shopping carts around in a long, inexplicable conga line without ever actually buying anything.
(Seriously, a day when I can use my Wal-Mart tag and my Zombies! tag together? Awesome!)