Cigarettes, alchohol, certain or all foods (I've gone a week before, if you decide to do something like that, just make sure you drink plenty of water), internet, some or all electronic devices, electricity (I've done this for a few weeks), running water (14 days straight is my record), hot water, and refrigeration (apperantly), are some good examples.

If you really want to give up something meaningful, throughout the day just consider all your actions and decide what things you are most dependant on.