Disposable flashlights are a great thing to have on hand: they are a useful size, and extremely tough. The downside is that they can get pricey when you toss them after a couple of uses. Unfortunately, they can't be recycled, so every time you dispose of one, it ends up in a landfill.
Just because they are sold as single-use doesn't mean they have to stay that way. Next time you buy one of these $2 disposables, use it up and replace the batteries. They aren't manufactured for this, so it will take a bit of work to get it open. This Instructable has everything you need to replace the standard AA cells and extend your flashlight's life.
When you're closing it up, duct tape will do the trick and be easy to disassemble next time. If you want a polished look, you can easily weld it back together, then sand it down smooth.
The tutorial is done with an Eveready brand disposable ZAP flashlight. If you know how to change the batteries in any other brands, please share your tips with us!