Here's a wonderful exercise I learned from Dory Kanter in her beautiful book Art Escapes. It's called paint past the photo. Basically, you put a picture down in the middle of the page and paint past the edges. It's a wonderful way to practice color mixing and working on your ability to use a realistic style.
Here is one I did using a photo I took at Joshua Tree National Park. It was approaching sunset when I was there, and the colors and shapes of the rocks were amazing.
And this one I love, because the small person walking on the trail is my husband.
Thank you to Dory Kanter for this great idea!!