Be careful with your terminology here. There's no "activation", it's a triggered ability, not an activated one.
Trigger payments occur during resolution, unlike activated ability payment which occur before a spell hits the stack.
So we have:
0) You gain 1 life (from something that doesn't matter).
1)
Serene Steward triggers, targeting the only legal target -
Skulking Ghost.
2) With the SS trigger on the stack,
Skulking Ghost now triggers since it's AFRAID OF BEING LOOKED AT.
3) Both players pass priority, SG is sacrificed as it's trigger resolves.
4) Both players pass priority, SS trigger fizzles, since it's target is illegal (Ghost is no longer on the Battlefield)
In a case where the SS target is still on the Battlefield as it's trigger resolves, the user is asked to pay

now, and you can say no.
I believe Hanmac brought
Willbreaker up because it's a similar scenario:
0) You have a
Serene Steward and
Willbreaker on the battlefield. And gain 1 life (again, doesn't matter)
1)
Serene Steward triggers, you choose an opposing creature and put the SS trigger on the stack.
2) Your
Willbreaker triggers since a (triggered) ability you control has targeted an opposing creature.
3) Both players pass priority,
Willbreaker trigger resolves, you gain control of your opponents..
Siege Rhino.
4) Both players pass priority, SS trigger starts resolving, you can choose to pay

or not, but either way it won't affect the
Willbreaker trigger. If you have the mana, I hear 5/6 Rhinos are pretty good.
Make sense?