Polyoxymethylene (aka Delrin, acetal, polyacetal, and polyformaldehyde) and other similarly dense thermoplastics can be used for many substitute training weapons including, but not limited to (in my personal experience), bokken, bo, jo, hanbo, training knives, jitte, kunai, and training shuriken (with thin plates). For staves, it's often best to order them pre-cut to size and use a buffing wheel on a rotary tool to round the edges (the friction heats the plastic and creates a melt removing the sharp edges. Alternately, scrap carpet may be used).
Aluminum could conceivably be used, though it's recommended that you use a tube rather than a rod to eliminate weight, welding a cap to the end to create the appearance of a rod. Aluminum is flexible and light, and will likely dent.
For body contact, we in the Bujinkan often use fukuro hanbo, which consist of a core – either a bamboo rod or pvc pipe – wrapped with leather and light padding. This tool is dense enough to withstand throws, and gentle enough to do little more damage than a good bruising.
Anyone who has ever spent any time around boats or surfboards knows to stay away from fiberglass. Exterior layers of the material crack when forced to flex.
Ultimately, wood is the standard way to go. There's a vast variety, and having a sturdier (more perpindicular-compression resistant) wood than your opponent means your weapon will outlast theirs.