If you own any of the old, worn, rare Levi's now in demand in Japan, Europe, and other points foreign, don't part with them for less than the going rate, which seems to be headed toward astronomical. (I read in the paper that somebody paid $10,500 for a pair of 1927 Levi's Deadstock Buckle Backs.)
If, however, your worn trousers are not a valuable antique, you can turn them into useful carryalls. Denims make excellent bags with their pockets and interesting labels.
I have four tattered bags made years ago, three of denim, one of velveteen. On birding trips my husband sports one cut from his old trousers. These bags are easily sewn and will hold a small book, a few bucks, credit card, passport, airline tickets, and handkerchief. Often I have managed to stuff rain gear or a lightweight jacket inside.
The importance of this kind of carryall is that you don't have to leave it in the car or van as you might a purse. In birding spots in Ecuador, Africa, and Trinidad I have carried essentials in one of these bags bouncing softly at my hip at the end of a long strap.
Creation is mostly by eye. Cut material for the bag in one piece, using half the trouser top, front and back. Placing the pants on a flat surface and, taking your courage in hand, cut a chunk at least 13 inches long and 16 to 18 inches wide. This size allows for a seam on one side and at the bottom. The other side seam and the top are already finished, as well as two pockets. Keep the belt loops; they're convenient. Decoration can suit your personal style.
If you're using men's jeans, you can make a bigger carrying bag. Actually, all you really need is one that is deep enough for the top to stay closed by itself and wide enough at the top to drop a book through.
The strap, made from the pant legs, should be cut double width, plus extra for seams. You may need to piece it, because the strap must be long enough to cross your chest and go over your shoulder and across your back. The bag should ride comfortably on your backside. I make the strap, turn it right side out, press it, then pin it to the bag. Experiment with pinning, for the bag should hang level. The straps will be sewn to the bag at an angle. You will want the length to be right for your body size.
Many a traveler has envied the ragged bags I carry. I hope that yours will take you on a marvelous journey.