Perhaps Theseus' most famous adventure was his defeat of the Minotaur of Crete. When he arrived at his father's city, Aegeus was under obligation to deliver seven boys and seven girls every seven years to Minos of Crete, who would feed them to his wife's illegitimate monster child. (How a bull's head could chew flesh, I'm not sure. I'd have to suspect that a half-bull and half-human creature would be a vegetarian, but I guess there's no telling what will result from bizarre unions between people and bovines.) With help from Princess Ariadne, Theseus killed the Minotaur, and brought Ariadne with him on his voyage back to Athens. For some reason, though, he abandoned her on the island of Naxos. Some versions of the story go on to say that Dionysus rescued Ariadne and took her for his wife, a common sort of element often added to Greek stories of abandonment, perhaps to soften the blow. Regardless, when Theseus forgot to change the sail, his father assumed the worse, and drowned himself.
Theseus had several other adventures in his life, most of which involved killing, and which seem to be related in a somewhat different order on different websites. One of the craziest was when he and his friend Pirithous, Prince of the Lapinths (with whom he had fought a battle against drunken centaurs) decided they should marry daughters of Zeus. First, they abducted the young Helen (she of the face that would go on to launch a thousand ships and set fire to the topless towers of Ilium) from Sparta, and then went on to pursue Pirithous' choice of wife, Persephone. The only problem was that she was already married, and to the Lord of the Underworld at that. Now, I would imagine that most people, when the woman they want is married to one of the three major gods of the Greek pantheon, who lives in a shadowy, monster-filled domain from which escape is practically impossible, would just give up on the idea. But the two guys decided to go through with the plan, and ended up being trapped in chairs by Hades for twelve years. (You know, I have to wonder if this ill-fated adventure would make for a good buddy comedy. Theseus and Pirithous in: Road to Hades?) Herakles eventually showed up to rescue them. Well, at least he rescued Theseus; whether he could save Pirithous depends on which version of the myth you read. I'm not sure it matters that much, as his the Lapinth's story is pretty much over by now. The versions that say he was rescued from Hades have him being killed by a dog not long after that. Anyway, once Hades returned to the surface, he discovered that Helen's brothers, the Dioscuri, had taken her back to Sparta. I'm guessing that they found out how he had treated his LAST girlfriend.
The tales of the Athenian's screwed-up love life weren't over yet, however. While visiting the Amazons, he abducted their queen Hippolyta, and took her back to Athens with him. According to a certain late-sixteenth-century play, their wedding was accompanied by a version of Pyramus and Thisbe acted out by some rude mechanicals. Theseus and Hippolyta had a son named Hippolytus, but their love affair did not last long. Theseus' next wife was Phaedra, the sister of Ariadne. No, I'm not sure why she would agree to marry the guy who had abandoned her sister either, but maybe she didn't really have a choice in the matter. Anyway, she ended up falling in love with Hippolytus, which ended (not surprisingly) in tragedy. After Hippolytus rejected her advances, she hanged herself, leaving a note accusing Hippolytus of trying to rape her. Theseus then cursed his son, who died at the hands of the gods. Theseus himself was killed by Lycomedes of Skyros, who pushed him off a cliff into the sea.
In honor of the legendary hero of Athens, here's Black Francis' weird rap-rock take on the first journey of Theseus.
The Seus - Black Francis