Thank you for the reply!
I actually ended up changing the ambient color to a darker gray, and that seemed to help, as did changing the sun color. As it turns out, I think I'm going to use lighting for my level after all. I placed a bunch of lights in it yesterday, and it looked really good, much better than the "flat" look I originally had in mind. The lighting makes it look much more 3D, so for my "flat" map I think that will make it easier to tell what everything is.
Sure wish I had better options for taking high-rez in-level screenshots though...