Sell Portrait Sketches is a legal activity for earning money, and you make more money the higher your Art skill is. The base daily return is within $3-$18, on the higher-end of the scale for skilled artists. However, competition in liberal societies halves or quarters the return. It also boosts your art skill by 4 points. If you make a Formidable skill check (separate from money earned), your work also provides some background liberal influence.
Another activity, Spray Graffiti, can also max out your Art skill, as well as helping promote the Liberal Agenda through influencing public opinion; however, it is illegal. Once your Art skill is maxed out, Sell Liberal Art makes a lot more money.