Create a colorful design on your backpack using fabric paint or fabric markers. Wait for the designs to dry before you move onto other decorations.
Tie colorful ribbons to your zipper pulls to make your backpack pop. You can tie the ribbons into bows, or let them hang loosely. You might want to get some scissors to cut the ribbons to appropriate lengths.
Affix some fun, funky pins to your backpack to add more color and to show off your interests; there are pins out there for sports, bands and places.
Attach some colorful keychains to your zipper pulls to add more to your overall look and make it easier to pull your backpack's zipper. You can find photo keychains to show off your friends or sports pictures.
Iron on or sew some patches onto your backpack. Patches come in a variety of fun designs and they add some texture to your backpack.