A penitential act is an act of penance, of course!
There are lots of things to choose from in deciding upon a penitential act: sleep on the floor, take an ice cold shower, put a pebble in your shoe, no candy or snacks, an extra hour of prayer in front of the Blessed Sacrament, walk to work instead of drive, etc., etc. There’s lots of room for creativity!
I usually go without listening to music in the car on Fridays as my act of penance. It’s much more of a penance for me than not eating meat.
Of course, now my bishop is inviting the Catholics of my diocese to return to abstaining from meat on
every Friday, even after Lent is over. So I’ll be joining you in meatless Fridays year-round.