Design for a Cause
A campaign to bring more meaning to your passion and more benefit to the society.
If you are familiar with the Run for a Cause campaigns, you are already familiar with Design for a Cause. Literally, just replace 'Run' with 'Design'.
The idea is to bind a social purpose to your design projects.
For instance, if you want to enhance your portfolio by conducting a UX Analysis for a random website for free, let it be for a website that has a strong social purpose (e.g. charity website).
To have even more impact with your pro-bono work, start a fundraising campaign for a charity and ask your connections to 'sponsor' your work by donating to the campaign. That way you can also engage your community in a good cause.
Take this idea as a skeleton and apply your own creative flare to it!