Andrew,
This is a great question. Your second answer was more in line with a sermon. That is wonderful and quite appropriate from the pulpit. While I am not a huge advocate of systematic evangelism programs,
the Way of the Master does a good job of using the decalogue to overcome the objections to original sin. While you don't have to duplicate WOTM, you can effectively use the decalogue to cause an individual to recognize their sinfulness. WOTM does it this way:
Christian: Have you ever told a lie?
Sinner: Yes, of course. Everyone has.
Christian: What does that make you?
Sinner: A liar.
Christian: Have you ever take anything that doesn't belong to you?
Sinner: Yes.
Christian: What does that make you?
Sinner: A thief.
Christian: Then if we use the Ten Commandments as God's standard, by your own admission you are a lying thief. If God uses the Ten Commandments as the standard by which He will judge the world, would you be innocent or guilty?
Sinner: Guilty.
You see the progression? I have had people tell me that they have kept the Ten Commandments but after questioning them this way very few of them would be audacious enough to claim they are innocent.
I want to be careful here. To the unregenerate sinner the doctrine of original sin is not necessary for salvation. The understanding that they have sinned and are under God's wrath is necessary. I wouldn't get hung up on a discussion of original sin as a general rule with an unbeliever. If the conversation allowed I would certainly mention it, but I would try to make sure that the individual understood they they have sinned and are in need of forgiveness.