Like Velonomad says unloading trucks is a good way to make a fast buck. At many warehouses you'll see "lumpers" outside the gate soliciting truckers to either unload the truck for them or help them unload. You could make anywhere from $30 (if the trucker helps you) to $75 or more depending on the freight for just one truck. It'd take about 2 - 3 hours if it's a floor load. You'd probably get paid cash.

One thing's for certain, you'll sleep well after doing that!