Topics such as these ones are the best:
What are you listening to? (Music)
What’s your favorite music genre?
How old are you?
What’s your favorite drink?
What’s your favorite fruit?
How much sleep do you get?
How old are you?
How did you find this site?
Do you have a cellphone?
Those are the main topics I post on forums that I’m engaging in a post exchange with - if they don’t have them already.