Fjallraven backpack mini
 

The Kanken comes in several fun colors. But be sure not to leave it in direct sunlight, or it might fade.

Fjallraven backpacks are made with recycled materials and eco-friendly materials, so you can rest assured that you're doing your part for the environment. Moreover, they have a strong reputation for durability and dirt resistance.

You can even buy one online from places like REI. Some models have a handy boarding pass compartment, making them a good choice if you're traveling by plane.