I, too, am an apartment dweller constantly seeking ways to green my space. I've amassed quite a collection of succulents that love the mid-day, direct sun my windows offer, but also don't mind the indirect light of early morning and late afternoon. They're drought tolerant, so if I miss a watering, they don't complain and most of them are hardy to 20F. I bring them inside in the winter, although in warmer climates they could stay out all year. Ferns are great for just out of the direct light of the windows and moss (in a shady terrarium) smells wonderful and, once established, is super low-maintenance. Just a few ideas to get you started.