Mainly it can be something that you idolize more than God. As I said, for atheists or other religions it could be about reevaluation, or just taking a break from something. As you said, most people give up sodas, chocolate, sweets, or snacking. Sometimes you don't have to give up anything, you can add something. For example, being nicer to people, conserving energy, taking quicker showers. All these could be factored into Lent.