I heard that the Nightblade ability mark Target can be used without breaking stealth, are there other activated abilities that can be used without breaking stealth? My hope would be that buffs that don't deal damage would fall into this category.

Once the nightblade fires a bow, he is pulled out of crouch. I didn't like it at first either, but you develop a skill early to place your character back into stealth, so it evens out.

Destro staff weakness to element. Probably you mean debuff that doesn't do damage.

EDIT: Now that i think about it. Weakness to element might break stealth but doesn't draw aggro.

Any self buff doesn't break stealth. Mark target is the only debuff that the nightblade can employ that doesn't remove you from stealth.