Use it for what it was intended for. Much of the active ingredient(s) will have dissipated within the container, but your lawn will still likely benefit from what remains of nutrients that are still lingering in the product. This is certainly the best alternative. Whatever you do, do not throw unused fertilizer or weed control products into the garbage since it will inevitably end up in the landfill, which is not where these products belong. If you have some other type of fertilizer or pesticide you need to dispose of, give it away to someone who might use it or you can call an environmental agency that properly disposes of petroleum products and other chemicals that pose a potential danger to the environment.