My longest penance is to say a decade of the rosary… for the rest of my life. This was from a visiting priest. I had only confessed venial sins, and it had only been 3 days since my last confession. (This was at a penance service.)
I have had to read Psalm 51 and the parable of the prodigal son before, both from the same priest. I think that reading Psalm 51 was the best penance I had ever had, but for other circumstances than just its beautiful and repentant words.
From my usual confessor, I normally am required to pray for the people who could be hurt by my sins. Like, if I confess speeding, I pray for people hurt and killed in car accidents. Or, if it’s being mean or disobedient to mom, I pray for familys who have trouble getting along. I really like my priest’s penances… they always fit so perfectly. And I can tell when he thinks I really messed up, because then I get the full rosary (20 decades) or something.