Leveling up in Skyrim is not a reward unless you have the skill ranks to compete at that level. Leveling the player without increasing their skills is significantly to their detriment, because enemies get tougher while the player doesn't improve at all. You might be better off letting the player pick, say, ten skills to automatically gain a rank in. If they pick their higher rank skills, they'll probably increase a level. If they pick their lower rank skills, they probably won't increase in level, but in that case they probably don't want to increase in level because they are trying to improve a low ranking skill to the point that it is viable at their current level before they go increasing level again, and the game world gets harder. If the bonus you are looking to give the player is the +10 in an attribute, I would suggest just giving them an ability that boosts that attribute, because again otherwise you're actually penalizing them for completing your quest.