My mom had a nice pair of pants with a built-in belt that she was no longer able to wear, because the belt buckle had broken. This fix hardly needs explanation, but I thought I would share the idea with you.
First, I removed the buckle from an old belt she didn't wear anymore, then I removed the stitching from the loop on the pants that held the old buckle.
I placed the "new" buckle inside the loop and stitched it closed.
That's it! A free fix for a pair of pants that would have otherwise gone in the donate box.