Any nuetral object large enough to fill the frame will work. The camera will then adjust the white balance so that it records as nuetral.
This being winter, snow would work well since normally it records as an annoying blue in the shade.
If you want to have some fun, lie to your camera that something blue is neutral (that is, manual white balance on it). This will cause it to add an orange cast to everything (sunsets might be interesting to experiment with) in a vain effort to lower the color temperature to compensate for the blue of your target (remembering that blue is actually warmer than orange).