I tromped around the grounds of the nearby forest preserve until I found an interesting tree-like shrub (or shrublike tree) with some kind of berries. I was looking for some combination of red and white to complement the bluebird that I was imaging painting. The leaves were almost completely gone, and the berries were still white, but kind of shriveled. I wanted it to look weathered. I find that plants are more interesting when they're not looking like they're at their best. I just happened to have a scissors with me in the car. With some effort, I was able to hack away at the shrub until I detached some branches.
It appears that the plant is a grey dogwood, judging from my googling.