Instantly leveling up thievery and assassination spam...

[Kai]

Find a weak enemy that's knocked out by a guard (everywhere) and buy a simple house that you can lock in a town (the hub) and then put the person down in there.
Make sure he has no weapons, and maybe equip him with armor and items that negates his ability to fight you... Then when he's 'normal' state you pause the game and loot him with your character, the ui will close when the item equipped to him (very very low chance) disappears, but the game is still paused and you can re-open the loot menu and attempt to loot his armor again.

Successful attempt gives you MASSIVE thievery bonuses.

To train assassination, basically do the same but unpause to let him stand up while you are stealthed behind him, and attempt to knock him out. Even if you can't, you can use non-lethal blunt weapons and first aid to knock him out and then repeat until you raise your chances.

You could do this to higher level enemies in theory, as their skills being higher than yours might see to much higher exp bonuses... Or maybe put rarer or expensive items in their inventory to 'steal' back and forth to cause the exp gained to be massive.