Go to High Hrothgar where the Gray Beards are, turn the difficulty up and hit the Gray Beards in the back while hidden. In the early levels you will get ONLY ONE LEVEL PER HIT!!!!! In the later levels you can get as much as SIX!!!!!! You can get you sneak to 100 in 10 minutes. Using the Thief Stone and the Marriage buff this will go up even faster.
Go to Falkreath and run to the Jarls house, there are two guards stationed outside the door, pick the guard to the right of the door and hide behind him in the corner with the bits of wood, enter sneak mode and wrap the controller cable around the pad so you are stuck backwards walking. Go have fun doing somthing else for a few hours. You can probably do this in a lot of places, but I know this location works well because the guard is extra vigilant in his guarding duties and does not wandor off.