I've seen a bunch of people online that have made their own watercolor travel palette with an altoids tin. Since I wanted to try out gouache (a kind of opaque water-soluble paint), I decided to make one of my own, and fill them with gouache!
Cool huh. This is what the trees looked like when finished:
In case you're wondering, here are the colors in my palette:
Here's another sketch in my sketchbook:
I really like how you can layer light over dark with gouache, I don't have to plan ahead as much.