Henry David Thoreau said that the only investment that never fails is goodness. How could you go about investing in goodness?
The first thought that comes to mind is "empathy." To invest in empathy, the ability for nonjudgmental and compassionate perspective-taking, is the good that would keep on giving and expand one's own horizons, as well. It's one thing that the world is sorely lacking, while there's an abundance of platitudes, presumption, and pats on the back.
I have my fair share of problems (more than, if I have anything to say about it!), and when I turn to friends I'm not looking for a magic cure for my childhood, my depression, or my fears. I'm looking for someone who sees that I'm in pain and who sees what I feel is ugliness, but reacts with understanding, curiosity, acceptance, and compassion. This is empathy, and it gives me strength and perspective - much more than simply being told what I should do, what I should believe, what I'm experiencing, or what I should feel.
Empathy is a kind of healing, and to learn to better provide it would certainly be investing in goodness. I sometimes worry I can't repay the kindnesses I've been shown.