While on your travels, come up with as many things as you can find to use as pigment (adding water if necessary).
Some examples include crushed berries, mud (using different kinds of dirt), crushed leaves, spices.
Alternate: Document an experience using stains.
Since I was in the kitchen, I decided to go with the alternate and document an experience using stains. The "stains" would be made from things in my fridge and pantry. I crushed up berries and spread them around, mixed spices with water and ground them into the canvas. I poured coffee and wine. Surprisingly, the coffee, wine and blueberries made the least color. The paprika and raspberries made the richest color. I have to say that it was the best smelling piece of art I've ever made. When it was done, it smelled like the dough for cinnamon rolls. Mmmm...cinnamon rolls. :)