I'm in exactly the same position as you I just got my art exam too!
I think contrast is a good one to pick, and you don't have to just use contrasts such as light and dark or night time.
You could also take an ordinary picture say like in trees in a forest, you have the shadow of the trees and the light coming through, then when you go to paint it use bright colours for the light areas with much darker in the shaded.
There's also contrasts between colours in plants that you could do, say pick red and bright green or oranges in plants or blue, red, yellow flowers and painting them at a higher saturation with a background of dusk or night time that would contrast this with a dark colouring and less activity going on in this area of the painting.
Or if you did want to do a night time scene perhaps a city at night? The lights would stand out against the sky and having the moon in the background would also give you a contrast.