We have all sorts of useless bags and cases at the office.
Ditto. Plus I have a bunch that got accumulated at conventions and such. I've never recycled the straps as weapons slings, but the bags and cases are useful for hauling stuff to and from the range. One of my main range bags is the carrying case for a circa-1988 Mac. It's padded, has sturdy zippers and internal pockets, and perfectly holds a stack of hard pistol cases. Other, more flimsy bags inevitably end up tearing or splitting a zipper...but they're free and there is always another one somebody wants to give me as an advertising premium.
Home Depot, Trader Horn, and other places that carry handyman stuff are full of things that are useful to shooters. Check out the gloves and belts, for example. They're much cheaper when sold as work gloves than as shooting gloves. I have a metal briefcase-style toolbox from Home Depot for which I paid about $25. The exact same case is sold at gun shows in this area as a "range box" for anywhere from two to three times as much.
Reactive, shattering targets are enormously fun. Littering the landscape with junk isn't. Instead of blowing up old electronics and crapping glass, plastic, and toxic metals all over the range, make some ice. Cardboard 1/2 gallon milk cartons are the perfect size. Fill some with water, a little food dye if you like, and freeze. Take a cooler full of them to the range. Peel off the carton and you have a nice colorful block of ice that will shatter in the most gratifying manner when hit by something with a little oomph. I love busting them with an AK or my PSL. For reactive fun with a .22, cheap ginger snap cookies (or animal crackers if you're really good) bust nicely and don't linger. Produce can be fun too, but some clubs frown on it as the residue draws insects, especially from sugary things like watermelons.