Relationships,
Social Dynamics,
Situational things going on around you,
Current events,
The person(s) you are talking to, people naturally like to talk about themselves when given an opportunity to do so.
Anything that gets people into a positive state, thoughtful state, imaginative state or conjurs up a good debate.

Try and avoid closed questions or anything that can lower a persons state such as political or religious topics, unless you find a girl who happens to be passionate about such topics.