You can take strips of the strong, sturdy fabric (usually made of nylon, but may be blended with cotton or other natural fibers.) commonly used on packs and use them to reinforce the seams on your new pack or you can even sew them into new pockets and compartments on your new pack. Look at designs online and then create your own unique pack.

• Old day packs work well as tool bags to store circular saw, router, etc. They keep the shop dust off, are easy to grab and go mobile, and much simpler than the hard cases most tools come with.
• To reuse it as a pack, take it to a shoe repair shop to restitch straps and install a new zipper, if it needs one.
• Make it into something you can hang on the back of your door – as a misc. item holder.
• Use it to hold stuff in your house. The strap doesn’t really need to work if you’re using it to hold out-of-season clothes, toys, or extra plastic bags, etc.

Duct tape is strong and might fix a weak area of your pack.

If you don’t wish to keep the pack maybe someone else might have a use for it, or it's parts.

Here are a few customization ideas: http://matadornetwork.com/goods/9-ways-to-customize-your-backpack/