Depends. I would at least give an honest attempt at being a mage. Unlocking the arcane secrets and attempting to gain immortality and eternal youth. Might try finding Sheogorath, my constant swinging from Lawful Evil and Chaotic Good might just allow me to somehow convince him that if he were to give me eternal youth that I would be the source of a great many situations that he could derive entertainment out of.
Of course I would be the darkest mage Tamriel has seen rise in a long time. And I wouldn't have to even become a criminal, don't dress in necromancer robes, and don't let anyone see me using necromancy if I have to use it. I can just practice on the countless bandit hideouts that litter Nirn. Seal all of the exits in a way that would take a while to open unless you know exactly how to, and go to town with my horrible, dark, twisted experiments. Maybe even some research into eternal youth if I can't convince Sheogorath to help me.
If that didn't work out, I would probably go on to be a blacksmith. As a cover. I would in actuality be a thief. Breaking into the houses of those who do not share the same worldview as me. By that I mean those who are on the other side of the whole "civil war" thing. So I would probably hang out in cities that support Ulfric Stormcloak. Tons of houses to break into.
Steal from the rich and give to the poor. I would be a sort of Robin Hood figure. I would even have an iconic way of dressing. Black robes that do not hinder my movement, with a giant white eye painted on my chest and back. "Watcher." I would be called. Judge of the greedy and giver to those in need. I would then change clothes into a generic thief outfit, and steal back from the poor, keeping whatever I like for myself. Except for food I guess, and money if I can help it...Like I said, I switch between being Lawful Evil and Chaotic Good.
Probably going to give an honest to god try at being a dark mage though. Sounds way more fun. And hell, if I gain enough power, I might even raise an army of darkness to overthrow all known forms of government, and rule as the Overlord of Tamriel. Maybe even Nirn if my power and influence can spread far enough.