Get a pen, a pad of paper, and some Crayola markers! For instant fun, just add WATER!
Here are some sketches I've been doing to experiment with elementary school supplies. It is quite wonderful to discover that you can make cool art with cheapo supplies. I just used a ball point pen and Crayola washable makers. No Faber-Castell or Prismacolor here.
The markers work kind of like watercolors, only they are slightly more permanent. They are not as workable, but it's better than watercolor pencils in my opinion. It's also super simple if you want to do any sketching on location.