Well, there are different examples according to your lifestyle and your condition in life. And you have to ask God and your guardian angel to illuminate you as to which kind of penance you can do.
It could be some abnegation in eating or drinking. Some, it could be very discreet. It could be very small, for example, not to take sugar, not to take salt, and so on.
No one will observe this. It is better to make such penances that are hidden, not seen by others for example. Or to be awake 10 minutes earlier in the morning. This could be a small act of penance. And, not to assist television, maybe, and so on.
Sometimes it could be a penance to support a difficult person in your neighborhood, in your family. Or to keep silent, not to answer always. It could be a good salutary penance. So, I repeat, it depends on your conditions.