One of the more popular roof rack additions is a water tank. These are handy, but the water can get hot and they increase your centre of gravity. On the other hand, you can knock one up in a few hours with minimal expense. If you are chasing more information on water storage for your 4WD, check this out - 4WD water tanks.
The above setup was made out of bits and pieces from Bunnings, and then painted black. This helps the water to get hot in the sun, and you can wash your hands with warm water, or have a shower.