Sew a backpack!
This project is cheap and pretty easy. It should only cost about $20-25 maximum (as long as you own a sewing machine).
Sewing Machine and Thread
3x2 feet of fabric (I used canvas)
2x2 feet of accent color fabric
8 Large Eyelets
3 feet of Rope, Yarn, or Thick String
Bias Taping (optional but I recommend it for a cleaner look)