That's such an interesting question because I know that the topic would be different with the age group!

I'm a single dad to a 2 year old girl and we speak English at home. Normally my conversations with her revolve around topics she knows how to express verbally: holidays, animals, letters, shapes, and colors. With these topics I usually encourage her to further express herself and I normally coax her along the way. 

When I talk to my 10 year old nephew my conversations normally revolve around video games, school, his friends, and girls he likes. With him I try to encourage curiosity and more academic conversations about things he is studying in school.

For adults and late adolescents who are interested in learning a language I think a priority for them would be to express themselves so topics revolving around past, present, and future items: family, hobbies, career goals/aspirations, childhood, and expressing why they are learning the language in a more articulate manner than "I like learning languages" (obviously contingent on their level of the language).

I love talking about animals, hobbies, telling random stories, school, and after that  see where it goes, depending on the  person I am talking to.

If  I talk to  a person who's much older  than  me, I choose  cultural  topics (movies, music, concerts, local dishes, history). Then if he or  she doesn't mind diving into childhood,  I jump into the topics mentioned  above.  I am curious  how  other  people  see the world around them.