In Acts 2:38, which is the better reading, the Douay-Rheims 'do penance' or 'repent'?

'Penance' is basically Old English, so you find it in the Douay-Rheims, and that is fine, because it still carries the idea of the Greek word here, metanoia. Metanoia combines meta, meaning change, and nous, meaning mind, so it means a change of mind. As long as that ingredient is there, and penance includes it, you are fine, because if you are penitent you have already changed your mind from position A to position B, and now you are trying to make reparation by reversing or apologizing for or rebuilding what you did. In that sense penance is a pregnant word: it combines both the repentance and the reparation in one word, whereas 'repent' is an action word that does not necessarily include what you do to repair the situation. Some people forget that just saying 'I am sorry' is not enough; God will forgive you, but there is still a load to unload, which is why we go to Purgatory, because you can repent all you want and be forgiven, but you still have to make reparation for what you destroyed, until you pay the last penny. And that is why the Church gives us indulgences: the same Jesus who says you will not get out until you pay the last penny also says, say these prayers and I will give you a plenary indulgence. That is the mercy of Catholicism.