We often go on walks to catch some sunlight and get her some exercise.

You might look into full spectrum lights as well. My father thought he might have some seasonal affective disorder going on and bought one. Take it for what it's worth, but wiki has some citations in the treatment section discussing the effectiveness of such treatments. They're a bit expensive (depending on your definition cutoff), but may prove to be easy and effective. I've read some on forums who turn it one while they eat breakfast in the morning.