The traditional practices of Lent—fasting, prayer, reception of the sacraments, and almsgiving—are still encouraged by the Church. Fasting is even mandated on certain days. Far from being "negative," these are a means of escaping our self-preoccupation and entering into the mind of Christ. Through eating less, we feel some of the hunger he feels for souls, and we grasp our dependence on him for our very existence. Though prayer, we join in that perfect union of the Trinity. Through the sacraments, especially Eucharist and penance, we are drawn more closely into that union, and we obtain grace to overcome sin. Through giving away material goods, we empty ourselves in a small way as Jesus emptied himself totally on the cross; we are sharply reminded of how attached we are to trifles. All of these things make clear to us how desperately we need God’s redeeming grace.
The practice of denying ourselves some legitimate pleasure is also healthy in a culture of wanton self-gratification. It subtly catechizes us about our own weakness and the power of surrender to God.
As for penance being merely passive acceptance of trials, it is not. Certainly, we may unite all our sufferings, large and small, with those of Jesus, and there is great merit in that. Everyone suffers in this life, and "offering it up" is a fruitful use of that pain. That is not the same as assuming extra trials to make reparation for one’s own sins or those of others. One should never undertake severe penances without the permission of one’s confessor, of course.
The sister had a point, though. We can benefit spiritually by actively living our faith during Lent: performing corporal and spiritual acts of mercy, attending stations of the cross, going out of our way to encourage and praise others, inviting non-Catholics to Mass. It isn’t an either/or issue; it’s both/and.