In a way, Hephaestus is the only hardworking god on Mount Olympus, the only one who produces new items on a regular basis, not just participates in a myth or two and then spends the rest of his godly life basking in glory.
In general, the symbols of Hephaestus are the anvil, hammer, axe and tongs used in his forge, and he’s rarely seen without them. In fact, he had several workshops: one forge on Mount Olympus, among the other gods, where he never felt quite at home, because of his physical appearance and another forge in Lemnos, an island in the Aegean Sea, his adoptive home. Some stories say that yet another forge existed on the banks of the river Okeanos, next to the place where the sun started his journey every morning on the sky, and one more on Volcanus, an island close to Sicily, where the Cyclopes worked for him. (In the Roman tradition, Hephaestus is named Vulcan.)
The constant attribute of Hephaestus is the fire, tamed and used by the god as an instrument of creation. By association, Hephaestus became a god of volcanoes, the mountains bearing fire.
Other symbols of Hephaestus include his sacred animals, dogs and donkeys. The donkeys are related to one of his myths: upset with his mother, Hera, for having rejected him at birth, Hephaestus made her a beautiful, but cursed present: a throne which would chain anybody sitting on it. Hera was trapped, and the other gods could do nothing to help her out, so they begged Hephaestus to return to Mount Olympus, and they promised him all the honors fit for his statute, as long as he set his mother free. Hephaestus, however, didn’t want to hear about Olympus anymore, took refuge in Lemnos and ignored the gods’ pleas. Then Dionysus, the god of wine, intervened: he paid Hephaestus a visit and got him drunk, then loaded him on a donkey and brought him back to Olympus. This is why Hephaestus is often depicted riding a donkey, even if he has a golden chariot, like all other gods. Thinking of it, he was in fact the one who made all those fancy chariots.
Among the birds, the symbol of Hephaestus is the crane, for being a migrating bird – same as the god himself, who couldn’t find his place among his kind and kept traveling back and forth to his forges.