Instead of abstaining from meat on Fridays outside of Lent, I abstain from a snack food I usually eat and either pray the Divine Mercy Chaplet or take 20-30 minutes to read the Bible. Would these actions be a sufficient penance for Fridays outside of Lent?
Under the current discipline of the USCCB, you are not actually required to do anything on the Fridays outside of Lent, so whatever you choose to do is fine. See the link below for more information.
Are Meatless Fridays Still a Thing?** by Steven D. Greydanus