oak we planted in my backyard, and last fall I was having a lovely time trying to capture the color in the trees.
About two months ago, I bought a set of Caran D'Ache Crayons--this set. (I'm appalled--they are significantly less expensive from Amazon. I always forget to check there vs. the art supply companies; sometimes Amazon has very good prices, although they don't always have a great selection.)
As for trees--here are three different trees using watercolor and crayon. My favorite is the one at the left. It's one of my favorite combination of colors.
I am enjoying the crayons because they give more flexibility and intense color. They're water soluble and you can lift if you change your mind. Try a small set, or visit, and try mine!
I'm not too sure about the painting at right, but the painting below was an old watercolor that didn't look like much. I turned it into a landscape with the crayons, and now I rather like it. (Note that I usually like my paintings immediately after I paint them, and given a month or so completely reverse my opinion. A post on that later.)
If you've discovered something new for your art that you're enjoying, please share! I'd love to hear about it.